[improve][ci] Switch to Java 21 as default JVM version for CI

[lhotari]

Motivation

Java 21 has been switched in the Docker image in 3.3.0 with "PIP-324: Alpine image (#22054)":
https://github.com/apache/pulsar/blob/branch-3.3/docker/pulsar/Dockerfile#L61

Pulsar CI should also run on Java 21 by default.

Modifications

• Use Java 21 by default in CI
• Run a scheduled build on Java 17 with docker image using Java 17
• Add -XX:+ZGenerational to PULSAR_GC parameters by default when running on Java 21+
  • We don't yet have benchmarking results, but since deployments explicitly set PULSAR_GC, this change won't get used unless users update their deployment PULSAR_GC to include -XX:+ZGenerational.
• Cleanup remaining Ubuntu related properties from the Maven build.

Additional Context

Netflix Engineering has reported in a blog post that ZGenerational brings significant improvements for their use cases (not related to Pulsar):

Even after we saw very promising results in our evaluation, we expected the adoption of ZGC to be a trade off: a little less application throughput, due to store and load barriers, work performed in thread local handshakes, and the GC competing with the application for resources. We considered that an acceptable trade off, as avoiding pauses provided benefits that would outweigh that overhead.

In fact, we’ve found for our services and architecture that there is no such trade off. For a given CPU utilization target, ZGC improves both average and P99 latencies with equal or better CPU utilization when compared to G1.

Documentation

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.50%. Comparing base (bbc6224) to head (aff2d69).
Report is 612 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #23373      +/-   ##
============================================
+ Coverage     73.57%   74.50%   +0.92%     
- Complexity    32624    34463    +1839     
============================================
  Files          1877     1934      +57     
  Lines        139502   145132    +5630     
  Branches      15299    15870     +571     
============================================
+ Hits         102638   108124    +5486     
+ Misses        28908    28712     -196     
- Partials       7956     8296     +340     

Flag	Coverage Δ
inttests	27.75% <ø> (+3.17%)	⬆️
systests	24.50% <ø> (+0.18%)	⬆️
unittests	73.86% <ø> (+1.01%)	⬆️

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see 603 files with indirect coverage changes