Closes gh-23487

The use of LinkedCaseInsensitiveMap, going back to 3.0, makes it
unnecessary to iterate over keys which can cause
ConcurrentModificationException.

Closes gh-23460

Closes gh-23490

Previously DefaultWebClientBuilder always defaulted the ClientHttpConnector
with ReactorClientHttpConnector. This worked fine if reactor was used.
However, it would break if the user was trying to leverage Jetty.

This commit defaults to use Reactory Netty HttpClient if it is present. If
it is not present it then Jetty's HttpClient  is used if present.

Closes gh-23491

Closes gh-23463

Closes gh-23484

Prior to this commit, the reference documentation build with asciidoctor
would get the common "spring-docs-resources" as a dependency and then
use it when generating the docs.

As seen in #23124, this can cause problems since we'd like to
consistently resolve our dependencies. In this case, the
"spring-doc-resources" archive is not published on maven central since
it's not officially supported by the Spring team as an open source
project.

This commit updates the reference documentation build to get this
archive as a simple download task and avoid resolving it as a
dependency.

See gh-23282

Closes gh-23492

This commit ensures that the spring-framework-bom project is ignored
from the global configuration block. If not, many conventions and
dependencies are added to it and add noise to the published BOM.

See gh-23282

This reverts commit fb0d6187.

This reverts commit 1539ba89.

This commit reorganizes tasks and scripts in the build to only apply
them where they're needed. We're considering here 3 "types" of projects
in our build:
* the root project, handling documentation, publishing, etc
* framework modules (a project that's published as a spring artifact)
* internal modules, such as the BOM, our coroutines support and our
integration-tests

With this change, we're strealining the project configuration for all
spring modules and only applying plugins when needed (typically our
kotlin support).

See gh-23282

Due to a bug (or "unintentional feature") in JUnit 4, overridden test
methods not annotated with @test are still executed as test methods;
however, JUnit Jupiter does not support that. Thus, prior to this
commit, some overridden test methods in spring-core were no longer
executed after the migration from JUnit 4 to JUnit Jupiter.

This commit addresses this issue for such known use cases, but there
are likely other such use cases within Spring's test suite.

See gh-23451

See gh-23451

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.