WebClient, WebTestClient, and ServerResponse

The recently added body(Object) variant can be confused easily with
body(Publisher, Class) forgetting to provide the element type and
only running into the IllegalArgumentException at runtime.

See gh-23212

This commit moves the dependency management and test source files
related to integration tests to a dedicated module.
This allows us to focus the root project on building the Spring
Framework.

See gh-23282

This commit removes JDiff from the Spring Framework build and instead,
adds a Gradle plugin that configure JApiCmp tasks on the framework
modules.

Fixes gh-22942
See gh-23282

This updates brings full interoperability between Reactor and
Coroutines contexts.

Closes gh-22986

See gh-23467

* pr/23480:
  Fix typo

Closes gh-23480

See gh-23480

Closes gh-23479

The spring-build-src module does not contain any tests. Thus, we do not
need any test dependencies.

Closes gh-23467

Add `SpringJUnit5` checkstyle rule to ensure that JUnit 4 annotations
aren't accidentally used in new tests.

The "must not be public" rule has been suppressed since there are
quite a few tests that extend base tests from other packages.

Closes gh-23477

Prior to this commit, the "reactive matrix" contained a duplicate
argument set per parameterized client.

Closes gh-23476

Inspired by the fact that example.com is currently not reachable from
my network.

 Closes gh-23451

This commit migrates the spring-test test suite from JUnit 4 to JUnit
Jupiter where applicable.

Tests specific to our JUnit 4 and TestNG support remain written using
those frameworks. In addition, some tests are still written in JUnit 4
if they extend a test class that should not be migrated to JUnit
Jupiter.

See gh-23451

This commit removes the JUnit 4 dependency from all modules except
spring-test which provides explicit JUnit 4 support.

This commit also includes the following.

- migration from JUnit 4 assertions to JUnit Jupiter assertions in all
  Kotlin tests
- migration from JUnit 4 assumptions in Spring's TestGroup support to
  JUnit Jupiter assumptions, based on org.opentest4j.TestAbortedException
- introduction of a new TestGroups utility class than can be used from
  existing JUnit 4 tests in the spring-test module in order to perform
  assumptions using JUnit 4's Assume class

See gh-23451

This commit migrates the rest of Spring's test suite to JUnit Jupiter,
except spring-test which will be migrated in a separate commit.

See gh-23451

This commit migrates parameterized tests in spring-core using the
"composed @ParameterizedTest" approach. This approach is reused in
follow-up commits for the migration of the remaining modules.

For a concrete example, see AbstractDataBufferAllocatingTests and its
subclasses (e.g., DataBufferTests).

Specifically, AbstractDataBufferAllocatingTests declares a custom
@ParameterizedDataBufferAllocatingTest annotation that is
meta-annotated with @ParameterizedTest and
@MethodSource("org.springframework.core.io.buffer.AbstractDataBufferAllocatingTests#dataBufferFactories()").

Individual methods in concrete subclasses are then annotated with
@ParameterizedDataBufferAllocatingTest instead of @ParameterizedTest or
@test.

The approach makes the migration from JUnit 4 to JUnit Jupiter rather
straightforward; however, there is one major downside. The arguments
for a @ParameterizedTest test method can only be accessed by the test
method itself. It is not possible to access them in an @BeforeEach
method (see https://github.com/junit-team/junit5/issues/944).
Consequently, we are forced to declare the parameters in each such
method and delegate to a custom "setup" method. Although this is a bit
cumbersome, I feel it is currently the best way to achieve fine grained
parameterized tests within our test suite without implementing a custom
TestTemplateInvocationContextProvider for each specific use case.

Once https://github.com/junit-team/junit5/issues/878 is resolved, we
should consider migrating to parameterized test classes.

See gh-23451

This first commit for this issue:

- allows JUnit Jupiter to be used for all tests
- adds a dependency on mockito-junit-jupiter
- migrates tests in spring-core to JUnit Jupiter, except parameterized
  tests

The following script was developed in order to semi-automate the
migration process.

https://github.com/sbrannen/junit-converters/blob/master/junit4ToJUnitJupiter.zsh

See gh-23451

See gh-23282

This commit removes unused parts of the Gradle build:
* Gradle wrapper customization which should not be needed in recent
versions of Gradle (or can be replaced with options in the
gradle.properties file)
* the branch strategy configuration

See gh-23282

See gh-23282

This commit moves the compile configuration from the Gradle DSL to a
convention. This configuration is not changing often, and we're using
that opportunity to make the Java source compatibility a project
property so as to easily recent JDKs this on the command line.

See gh-23282

See gh-21778

See gh-21778

See gh-21778