Experiment Pulsar CI build and tests with Java 21

[lhotari]

• Use Java 21 by default in GitHub Actions workflows
• Switch to use temurin-21-jdk in docker images
• [improve][test] Upgrade Mockito to 5.6.0 for Java 21 compatibility ([improve][test] Upgrade Mockito to 5.6.0 for Java 21 compatibility apache/pulsar#21385)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5af821d). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #155   +/-   ##
=========================================
  Coverage          ?   73.27%           
  Complexity        ?    32576           
=========================================
  Files             ?     1888           
  Lines             ?   140278           
  Branches          ?    15415           
=========================================
  Hits              ?   102786           
  Misses            ?    29407           
  Partials          ?     8085           

Flag	Coverage Δ
inttests	24.25% <0.00%> (?)
systests	24.67% <0.00%> (?)
unittests	72.57% <0.00%> (?)

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