Temporarily update `spring-boot-starter-data-rest` to declare
the correct spring-plugin version.

Closes gh-16646

* gh-18321:
  Polish "Reference Spring Framework RSocket section + polish"
  Reference Spring Framework RSocket section + polish

Closes gh-18321

See gh-18321

Closes gh-18323

Closes gh-18319

See gh-18316

See gh-18258

See gh-18256

See gh-18255

See gh-18254

See gh-18253

See gh-18252

See gh-18251

Closes gh-18311

Closes gh-18318

Closes gh-18317

See gh-18315

See gh-18314

See gh-18313

See gh-18312

Closes gh-18301

Update `ImageBannerTests` to use `inHexacecimal()` to help with
failure logs.

See gh-18301

Closes gh-18233

Add support for a `spring.banner.image.pixelmode` property which can
be set to `block` to use unicode block characters when rendering image
banners.

Closes gh-18301

* pr/18264:
  Add ANSI 8-bit color image banner support
  Polish 'Add ANSI 8-bit color support'
  Add ANSI 8-bit color support

Closes gh-18264

Update `ImageBanner` and `AnsiColors` to optionally support 8-bit
color output.

See gh-18264

See gh-18264

Update ANSI property support to include an 8-bit (256 color) option.

See gh-18264

Closes gh-18299

Update `ApplicationContextRequestMatcher` to ensure that the `matches`
method is never called before `initialized`. This fixes an issue
accidentally introduced in commit 5938ca78 where concurrent calls
to `matches` could trigger unexpected errors due to the fact that the
second call proceeded before the `initialized` method had returned.

Fixes gh-18211

Fix an error in the `ApplicationContextRequestMatcher` javadoc.

Closes gh-17877

* gh-16973:
  Polish "Allow the user that runs the app to be specified via an env var"
  Allow the user that runs the app to be specified via an env var

Closes gh-16973

See gh-16973