See gh-12934

Fixes gh-12807

Fixes gh-12934

Closes gh-13075

Closes gh-13074

Closes gh-13073

Closes gh-13072

Closes gh-13071

Closes gh-13070

Closes gh-13069

Closes gh-13068

Closes gh-13067

See gh-13056

This commit restructures the Cache auto-configuration to avoid an early
init on CacheManager (and potentially all its infrastructure). Rather
than adding a dependency on the validator bean, this commit relies on
the fact CacheAspectSupport checks if a CacheManager is available in the
afterSingletonsInstantiated callback. In this case, a simple bean with
a postconstruct callback is enough.

Closes gh-13038

Closes gh-13060

This reverts commit f78fd74b.

See gh-13056

Closes gh-13055

* pr/13061:
  Polish "Improve Spring Session docs"
  Improve Spring Session docs

Closes gh-13061

See gh-13061

Upgrading the JDK 9-based build to use JDK 9.0.4 has revealed that
Gradle 4.0.x doesn't work with it as it fails to parse the 9.0.4
version number.

This commit disables building the Gradle plugin on Java 9 (as we
had already done for Java 10) until we decide what to do
(see gh-12333).

Closes gh-13059

Closes gh-13058