See gh-18847

Closes gh-18851

* pr/18848:
  Drop JDK 12-based CI

Closes gh-18848

See gh-18848

Closes gh-18850

* pr/18845:
  Test the Gradle Plugin against Gradle 5.6.4

Closes gh-18845

See gh-18845

Closes gh-18733

Closes gh-18737

Closes gh-18840

Closes gh-17261

Closes gh-18798

* pr/18837:
  Add reference to Armeria starter

Closes gh-18837

See gh-18837

* pr/18838:
  Polish

Closes gh-18838

See gh-18838

* pr/18833:
  Remove unnecessary log4j exclusion

Closes gh-18833

See gh-18833

This commit creates a new configuration property
`spring.codec.max-in-memory-size` which configures the maximum
amount of data to be buffered in memory by codecs (both client and
server).

This property has no default value - it will let Spring Framework handle
the default behavior, currently enforcing a 256KB for provided codecs.

Fixes gh-18828

Closes gh-18818

Closes gh-18824

Closes gh-18823

Closes gh-18822

Closes gh-18821

Closes gh-18820

Update `HealthEndpointConfiguration` to also include adapted reactive
health contributors when project reactor is on the classpath. Prior to
this commit, reactive contributors were only exposed in WebFlux
applications. This was a regression from Spring Boot 2.1 that we didn't
catch because all our own reactive contributors all have non-reactive
equivalents.

Closes gh-18805

This commit is a follow-up of a change in Spring Framework[1] to make
sure injection points that are expecting a specific bean by name use
a qualifier.

As a result of this change, MVC uses the dedicated MVC validator again
rather than the general one auto-configured by Spring Boot.

[1] https://github.com/spring-projects/spring-framework/issues/23887

Closes gh-18672

Closes gh-18802

Closes gh-18801

Closes gh-18800

Closes gh-18799

Closes gh-18797

Closes gh-18796

Closes gh-18795

Closes gh-18794

Closes gh-18793

Closes gh-18792