Fixes gh-8883

See gh-8884

This commit makes sure to honour the `spring.aop.proxy-target-class`
property if set by the user. Previously, the
`MethodValidationPostProcessor` was always configured to use cglib,
regardless of the value of that property.

Closes gh-8869

* pr/8650:
  Polish "Add the ability to disable the trace filter"
  Add the ability to disable the trace filter

Closes gh-8650

See gh-8650

* pr/8813:
  Polish "Fix http status code with unknown log level"
  Fix http status code with unknown log level

Closes gh-8813

This commit changes the http status code to 400 when an unknown log level
is specified.

Closes gh-8798
See gh-8813

* pr/8879:
  Polish

Closes gh-8879

See gh-8879

See gh-8879

* pr/8185:
  Polish "Clarify edge case docs on ConditionalOnClass"
  Clarify edge case docs on ConditionalOnClass

Closes gh-8185

Previously, as a result of the changes made in de50cfa2, an
application would fail to start on Google AppEngine as it prevents
the creation of new threads.

This commit updates OnClassCondition so that it falls back to
performing the work on the main thread when its unable to shift it
to a separate thread.

Closes gh-8584

When used as a meta-annotation the value() attribute of
@ConditionalOnClass will fail silently resulting in the @Conditional
nature of the annotation being ignored.

See gh-8185

As `@TestComponent` is an implementation detail at this point really, this
commit removes any reference to it from the doc.

Closes gh-8421

This commit improves the lifecycle of the `NettyWebServer` and allows
multiple restarts when using devtools. Previously, the lifecycle was
tailored for a single start/stop cycle.

Fixes gh-8771