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-26682

See gh-26675

Closes gh-26675

Closes gh-26676

We now support Oracle's bind marker scheme that identifies dynamic
parameters using names that are prefixed with a colon such as
`:P0_myparam`.

Closes gh-26680

See gh-26662

Closes gh-26636

Closes gh-26662

* pr/26678:
  Polish "Fix Commons FileUpload URL in reference guide"
  Fix Commons FileUpload URL in reference guide

Closes gh-26678

See gh-26678

See gh-26678

* pr/26673:
  Update copyright year of changed files
  Polish

Closes gh-26673

See gh-26673

See gh-26673

See gh-26442

See gh-26442

Introduced 'idleReceivesPerTaskLimit': also mark task idle when idle receives per task threshold is reached
Closes GH-26195

Prior to this commit, @DateTimeFormat only supported a single format
for parsing date time values via the style, iso, and pattern attributes.

This commit introduces a new fallbackPatterns attribute that can be
used to configure multiple fallback patterns for parsing date time
values. This allows applications to accept multiple input formats for
date time values.

For example, if you wish to use the ISO date format for parsing and
printing but allow for lenient parsing of user input for various
additional date formats, you could annotate a field or method parameter
with configuration similar to the following.

    @DateTimeFormat(
        iso = ISO.DATE,
        fallbackPatterns = { "M/d/yy", "dd.MM.yyyy" }
    )

Closes gh-20292

Closes gh-26655

Closes gh-26656

See gh-26601

This commit skips the Bintray-related tasks in our CI pipeline and
instead relies on Maven Central for publishing Spring Framework
artifacts.

This commit also updates the CI pipeline to sign the artifacts directly
with the `artifactory-resource`.

Closes gh-26654

See gh-26638

* pr/26651:
  Polish "Update reference to deprecated CronSequenceGenerator"
  Update reference to deprecated CronSequenceGenerator

Closes gh-26651

See gh-26651

See gh-26651