Closes gh-26767

See gh-26767

Upgrade to Hibernate ORM 5.4.30, Tomcat 9.0.44, FreeMarker 2.3.31, Log4J 2.14.1, TestNG 7.4, HtmlUnit 2.48

Closes gh-26650

Let's wait Spring Boot 2.5 to upgrade to 1.1.x

Prior to this commit, the Spring Framework build would rely on
setting a custom Java HOME for building all sources and tests
with that JDK.

This approach is not flexible enough, since we would be testing
the source compatibility against a recent JDK, but not a common
case experienced by the community: compiling and running
application code with a recent JDK and the official, JDK8-based
Framework artifacts.
This method is also limiting our choice of JDKs to the ones
currently supported by Gradle itself.

This commit introduces the support of Gradle JVM Toolchains in
the Spring Framework build.

We can now select a specific JDK for compiling the main
SourceSets (Java, Groovy and Kotlin) and another one for
compiling and running the test SourceSets:

`./gradlew check -PmainToolChain=8 -PtestToolchain=15`

Gradle will automatically find the JDKs present on the host or
download one automcatically. You can find out about the ones
installed on your host using:

`./gradlew -q javaToolchains`

Finally, this commit also refactors the CI infrastructure to:

* only have a single CI image (with all the supported JDKs)
* use this new feature to compile with JDK8 but test it
against JDK11 and JDK15.

Closes gh-25787

Closes gh-26649

Upgrade to Hibernate ORM 5.4.29, Jackson 2.12.2, Jetty 9.4.38, Jetty Reactive HttpClient 1.1.6, R2DBC Arabba-SR9, JRuby 9.2.16, SnakeYAML 1.28, Checkstyle 8.41

Closes gh-26548

Closes gh-26545

Upgrade to Netty 4.1.59, Tomcat 9.0.43, Undertow 2.2.4, Mockito 3.7.7, HtmlUnit 2.47.1, Joda-Time 2.10.10, RxJava 2.2.21 and 3.0.10

Closes gh-26543

Closes gh-26510

Closes gh-26367

See gh-26367

Closes gh-26255

Upgrade to Hibernate Validator 6.1.7, Jetty Reactive HttpClient 1.1.5, Joda-Time 2.10.8, XMLUnit 2.8.1

See gh-26132

Closes gh-26176

Closes gh-26226