Fixes Java Examples ARM GHA on Java8

[Abacn]

Follow up of #28969

After the change the PostCommit Java Example ARM failing due to

A problem occurred evaluating project ':runners:google-cloud-dataflow-java'.
> Unknown Java Version 8 for setting additional java options

This PR

• Skips unsupported ver for -PtestJavaVersion option. This fixes Java Examples ARM GHA on Java8

this preserve the original logic before #28969 that is no-op when -PtestJavaVersion=8 is set. This happens to be configured in PostCommit ARM.

• Fixes missing version matrix element in readme.md

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[Abacn]

Tested by run the equivalent command locally:

./gradlew :runners:google-cloud-dataflow-java:arm:examplesJavaRunnerV2IntegrationTestARM -PtestJavaVersion=8 -Pjava8Home=.../.sdkman/candidates/java/8.0.322-zulu -Pcontainer-architecture-list=arm64 -Ppush-containers -Pdocker-repository-root=us.gcr.io/apache-beam-testing/github-actions -Pdocker-tag=java21test -PdisableSpotlessCheck=true -PdisableCheckStyle=true -PskipCheckerFramework

passed jobIds

2023-10-24_08_41_00-10801098760830802204
2023-10-24_08_41_03-14774137446205140215
2023-10-24_08_41_05-8911072195670644574
2023-10-24_08_41_12-7715440097359109658

Note that "-PtestJavaVersion=8 -Pjava8Home=...." are not necessary for java8 test (but we tolerate the redundant config as Java version tests are structured in this way).

[codecov]

Codecov Report

Merging #29117 (715bbf0) into master (bc06581) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #29117   +/-   ##
=======================================
  Coverage   38.37%   38.37%           
=======================================
  Files         686      686           
  Lines      101689   101689           
=======================================
  Hits        39021    39021           
  Misses      61089    61089           
  Partials     1579     1579           

Flag	Coverage Δ
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[Abacn]

Testing Java21 on ARM has a few blockers under the current GHA setup (run everything in single Java version) . I have a working branch trying to resolve them, but currently blocked by shadow plugin not yet support java21: Abacn@7aff7b5

This essentially means beam does not yet support compiling on Java21. We need to restructure the ARM postcommit to be compiled with Java8 (beam) + Java21 (test) as what I just did for JPMS. Leave this as future task.

[Abacn]

R: @tvalentyn

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[tvalentyn]

Thank you!

[Abacn]

Run Java_Examples_Dataflow_ARM PostCommit 8

running: https://github.com/apache/beam/actions/runs/6630847726/job/18013126134

update: passed, merging for now