Prior to this commit, the HTTP/2 server auto-configuration for Jetty
would require Conscrypt as a hard dependency.

This commit updates the auto-configuration for more flexibility and now
allows the following deployments:

* JDK9+ with the JDK ALPN implementation
* JDK8u252+ with the backported ALPN implementation
* Conscrypt with no JDK requirement

The auto-configuration now improves detection and guides developers in
case there is a missing `jetty-alpn-*-server` dependency.

The reference docs in the HOWTO section has been updated accordingly.

Closes gh-22188

Closes gh-22819

Previously, the project version was included in the name of the
Ant-built jar and the integration test assumed that there would be a
single jar in the output directory. This assumption did not hold true
if the project's version had changed and the project had been built
again without a clean. This resulted in two jars, one for the previous
version and one for the current version, in the output directory. This
caused a test failure.

This commit updates the build.xml to remove the version from the name
of the Ant-built jar and updates the integration test to find it.

Closes gh-22782

Closes gh-22780

Closes gh-22774

Closes gh-22490

Closes gh-22766

Closes gh-22767

Closes gh-22763

Closes gh-22758

Closes gh-22281

Closes gh-22754

Closes gh-22752

Closes gh-22755

Closes gh-22748

Closes gh-20920

Closes gh-22746

Previously, the endpoints' responses could occasionally exceed
WebClient's in-memory buffer limt, for example if the threads endpoint
was reporting a large number of threads or the threads had large
stacks.

This commit disables WebClient's in-memory buffer size limit so that
the tests passing is not dependent on the size of the endpoints'
responses.

Closes gh-22743

* pr/22740:
  Polish

Closes gh-22740

See gh-22740

Closes gh-22737

Previously, waitsForQuietPeriod would iterate 10 times, touching a new
file and then sleeping for 100ms at it did so. With a quiet period of
200ms, this was intended to result in a single change set containing
10 files. However, the test would fail occasionally as multiple change
sets were detected. The test is multi-threaded and is, therefore, at
the mercy of the scheduler. If the thread that is iterating and
touching the files takes over 200ms to be scheduled – exceeding the
watcher's quiet period – the watcher may detect a change set while the
changes are still being made. Eliminating this possibilty would require
the test to participate in the watcher's synchronization, which would
require some changes to its implementation. Instead, this commit
aims to avoid the problem by sleeping for 1/10 of the time (10ms) and
expecting a single change set of 100 files. The hope is that the much
shorter sleep time will result in the file touching thread being
scheduled well within the 200ms quiet period.

Closes gh-22732

Prior to this commit, an entry in the environment map provided to the
build plugin image building goal or task that had a null value would
result in a failure with a message that was difficult to diagnose.

This commit treats env map entries with a null value as an empty
entry to prevent the failure and also make it easier to provide an
explicit empty entry in the Maven XML.

Fixes gh-22703

See gh-21388

Closes gh-22726

Closes gh-22717

Closes gh-22723

Closes gh-22717

Closes gh-22721

* gh-21388:
  Polish "Test the launch script when executed directly"
  Test the launch script when executed directly

Closes gh-21388

See gh-21388