[improve][ci] Parameterize CI build so that Java 21 can be selected for manual runs

[lhotari]

Motivation

When preparing for Java 21, it is useful to be able to run the Pulsar build and tests with Java 21.

Modifications

Parameterize the JDK version in the GitHub Actions Pulsar CI workflow so that Java 21 can be selected when triggering the workflow manually:

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[lhotari]

Examples runs with Java 21 in my fork:
Pulsar CI
https://github.com/lhotari/pulsar/actions/runs/6577911040
Pulsar CI Flaky
https://github.com/lhotari/pulsar/actions/runs/6577912350

[codecov-commenter]

Codecov Report

Merging #21400 (a1d15e3) into master (700a29d) will increase coverage by 36.60%.
Report is 5 commits behind head on master.
The diff coverage is 62.50%.

@@              Coverage Diff              @@
##             master   #21400       +/-   ##
=============================================
+ Coverage     36.69%   73.29%   +36.60%     
- Complexity    12228    32578    +20350     
=============================================
  Files          1711     1888      +177     
  Lines        130684   140298     +9614     
  Branches      14242    15420     +1178     
=============================================
+ Hits          47957   102837    +54880     
+ Misses        76382    29378    -47004     
- Partials       6345     8083     +1738     

Flag	Coverage Δ
inttests	24.17% <0.00%> (+0.08%)	⬆️
systests	24.75% <0.00%> (+0.06%)	⬆️
unittests	72.59% <62.50%> (+40.69%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...dbalance/extensions/ExtensibleLoadManagerImpl.java	77.75% <100.00%> (+75.87%)	⬆️
...xtensions/channel/ServiceUnitStateChannelImpl.java	85.25% <100.00%> (+84.71%)	⬆️
...ache/pulsar/broker/namespace/NamespaceService.java	72.25% <ø> (+28.86%)	⬆️
...ava/org/apache/pulsar/broker/service/Consumer.java	86.23% <100.00%> (+23.58%)	⬆️
...org/apache/pulsar/broker/service/Subscription.java	69.23% <ø> (+61.53%)	⬆️
...rvice/nonpersistent/NonPersistentSubscription.java	53.53% <ø> (+25.25%)	⬆️
...ker/service/persistent/PersistentSubscription.java	76.24% <ø> (+26.39%)	⬆️
...sar/broker/service/persistent/PersistentTopic.java	79.34% <100.00%> (+27.30%)	⬆️
.../loadbalance/extensions/manager/UnloadManager.java	81.63% <40.00%> (+81.63%)	⬆️
...rg/apache/pulsar/broker/service/BrokerService.java	80.90% <33.33%> (+25.28%)	⬆️

... and 1444 files with indirect coverage changes

[Technoboy-]

Good approach to experience with Java21

[tisonkun]

Thank you!