Closes gh-10616

* pr/10615:
  Use properties in Security OAuth2 Client doc config example
  Fix Security OAuth2 Client documentation

Closes gh-10615

This commit fixes incorrect section level and sample configuration properties.

See gh-10615

Also, move properties corresponding to management server under
`management.server.*`.

Closes gh-10230

This commit adds support for Spring Boot error conventions with WebFlux.
The Spring MVC support for that is based on an `Controller` that's
mapped on a specific `"/error"` path and configured as an error page in
the Servlet container. With WebFlux, this support leverages a
`WebExceptionHandler`, which catches exceptions flowing through the
reactive pipeline and handles them.

The `DefaultErrorWebExceptionHandler` supports the following:
* return a JSON error response to machine clients
* return error HTML views (templates, static or default HTML view)

One can customize the error information by contributing an
`ErrorAttributes` bean to the application context.

Spring Boot provides an `ErrorWebExceptionHandler` marker interface and a
base implementation that provides high level constructs to handle
errors, based on the Spring WebFlux functional flavor.
The error handling logic can be completely changed by providing a custom
`RouterFunction` there.

Fixes gh-8625

Closes gh-10515

Closes gh-10651

* gh-10635:
  Fix typo in documentation

- As error handling sample is using `CustomErrorType`, fix
  description which mentions `CustomerErrorType`.

Previously, the custom layout sample did not configure the source
and target for the compiler. This meant that both defaulted to 1.5
which is not supported by Java 9's javac. The oldest support version
is 1.6.

This commit sets the maven.compiler.source and maven.compiler.target
properties to 1.8 (the lowest version support by Boot 2.0) allowing
the sample to build with both Java 8 and Java 9.

See gh-10515

See gh-10515

See gh-10515

Closes gh-10632

Closes gh-10631

Closes gh-10630

Closes gh-10629

* gh-10588:
  Polish "Provide informative reason when rejecting request with invalid level"
  Provide informative reason when rejecting request with invalid level

See gh-10588

Previously, bad request with no reason was included in the response.
This commit introduces the reason when invalid log level is sent in the
request.

Fixes gh-10588

See gh-9316

See gh-10587

* gh-10554:
  Honor logging.path config in filePattern for Log4j2 RollingFile

Previously, rolled over files were always written to logs/. This
commit ensures that rotated logs are written into the directory
specified in the application config's logging.path property.

Closes gh-10554

Closes gh-10606

See gh-9316

See gh-9316

See gh-9316

Fixes gh-10608