Issue: SPR-13344

This commit also drops the now-unused CUSTOM_COMPILATION test group and the outdated JavaVersion class.

Issue: SPR-10423

Issue: SPR-11569

Issue: SPR-13575

Some reactor modules such as reactor-netty were migrated under a new
namespace `io.projectreactor.ipc`.

xmlunit 2.1.0 is the latest release for xmlunit.
Most of the xmlunit functionality used within spring-framework
was done through the xmlunit 1.x helper class
`org.custommonkey.xmlunit.XMLAssert`.

As of xmlunit 2.0.0 most of the XML comparison methods are done
through hamcrest matchers exposed by the xmlunit-matchers
library. In some cases during the migration, the matchers
had to be customized with custom `NodeMatcher` or
`DifferenceEvaluator` instances in order to keep the assertions
correct (they were performed with xmlunit 1.x previously).

Issue: SPR-14043

This commit adds support for Google Protobuf 3.0.0 and make some changes
in the additional formats support:
* "com.googlecode.protobuf-java-format:protobuf-java-format" is no
longer required and its required version has been raised to 1.3+
(this lib adds support for JSON, XML, HTML formats)
* "com.google.protobuf:protobuf-java-util" is also now supported for
JSON format

Issue: SPR-13589

Issue: SPR-13344

Issue: SPR-14467

 - Added Woodstox before Aalto in the spring-web build, so that Woodstox
   is used as StAX implementation, and not the less featured Aalto.
 - Hardcoded Aalto dependency in XmlEventDecoder, instead of relying on
   the StAX XMLInputFactory.

Issue: SPR-14431

This commit removes `GuavaCache` and support classes. Caffeine supersedes
the caching support in the Google Guava library with an actively maintained
Java 8+ version in standalone form.

As it is the only Guava feature Spring framework integrates with, this
commit removes effectively any reference to Guava.

Issue: SPR-13797

Issue: SPR-13575

Issue: SPR-13294

Issue: SPR-13793

This commit also renames WebSocketIntegrationTests to WebSocketHandshakeTests.

Issue: SPR-14328

This commit adds a test runtime dependency on log4j 2 for every project
and migrates all log4j.properties files to log4j2-test.xml files.

Issue: SPR-14431

Issue: SPR-14429

This commit also removes outdated support classes for Oracle, GlassFish, JBoss.

Issue: SPR-14429

Commit 09008088 resolved the previous issue by moving the warning
suppression from the method parameter to the method itself.

Issue: SPR-13188

This commit also fixes broken javadoc links and code references.

Issue: SPR-13188

This commit introduces initial support for JUnit Jupiter (i.e., the new
programming and extension models in JUnit 5) in the Spring TestContext
Framework.

Specifically, this commit introduces the following.

- SpringExtension: an implementation of multiple extension APIs from
  JUnit Jupiter that provides full support for the existing feature set
  of the Spring TestContext Framework. This support is enabled via
  @ExtendWith(SpringExtension.class).

- @SpringJUnitConfig: a composed annotation that combines
  @ExtendWith(SpringExtension.class) from JUnit Jupiter with
  @ContextConfiguration from the Spring TestContext Framework.

- @SpringJUnitWebConfig: a composed annotation that combines
  @ExtendWith(SpringExtension.class) from JUnit Jupiter with
  @ContextConfiguration and @WebAppConfiguration from the Spring
  TestContext Framework.

Issue: SPR-13575

Issue: SPR-13188