Update the executable jar appendix to include details of the index
files.

Closes gh-20859

Update the `classpath.idx` format to align with `layers.idx` and allow
third-parties can parse it as YAML

Closes gh-20861

Update the `layers.idx` format so that it is more compact and can be
parsed by third-parties as YAML.

Closes gh-20860

Closes gh-20863

Closes gh-20862

This is done by checking for existing issues only in the milestone
corresponding to the current branch. For issues, the milestone is assigned
at the time of issue creation. For PRs, since the branch is different the
milestone can be derived from that while triaging the issue.

Closes gh-20680

This commit adds examples for configuring the default builder to use
an HTTP/HTTPS proxy when building OCI images using the Maven or Gradle
plugin.

Fixes gh-19984

This commit combines index adoc files for single-page HTML and PDF
into one file to remove duplication. It also renames index files to
reflect the separation between source format and generated output
format.

Finishes gh-20829

* pr/20851:
  Change call to createClassLoader(URL) from 'super' to 'this'

Closes gh-20851

ExecutableArchiveLauncher.createClassLoader(Iterator) calls
createClassLoader(URL) method with 'super'. This means overriding
createClassLoader(URL) is not possible without also overriding
createClassLoader(Iterator). The switch to 'this' enables that.

See gh-20851

This commit upgrades the Wavefront metrics export auto-configuration to
provide a `WavefrontSender` if necessary and use that to export metrics
rather than the http client Micrometer used previously.

As a result, the "read-timeout" and "connect-timeout" properties are no
longer honoured.

Closes gh-20810

See gh-20833

Closes gh-20853

Closes gh-20852

Closes gh-20850

Closes gh-20838

StatsD no longer publishes metrics about itself and StatsDMetrics is
deprecated as a result. This commit removes the auto-configuration of
it.

Closes gh-20836

See https://github.com/micrometer-metrics/micrometer/pull/1909

Closes gh-20837

This commit exposes an additional property for Graphite that allows to
restore the previous default behaviour with regards to tags, i.e.
prefixing the ones defined by the "tagsAsPrefix" property.

Close gh-20834

See gh-20833

See gh-20833

* pr/20819:
  Add reference to quickfixj spring boot starter

Closes gh-20819

See gh-20819

See gh-20835

Closes gh-20845

This property is a left-over and was never used in Micrometer so this
commit deprecates its use so that it can be removed in the next feature
release.

Closes gh-20835

Closes gh-20844

This commit fixes the AtlasProperties hierarchy so that it no longer is
a StepRegistryProperties. The AtlasConfig on the Micrometer side of
things does not share the common config hierarchy either and some
properties have different default and lifecycle.

Closes gh-20843

This commit moves Maven plugin content from several sections in the
main Spring Boot reference documentation to the plugin-specific
documentation.

Fixes gh-19165

This commit renames the PDF documentation files generated for the
build plugins when the distribution archive is created.

See gh-20829

Closes gh-20524