[Abacn]

Closes #35905 earlier discussion https://lists.apache.org/thread.html/bgkg1rpnrq37qztxn2vf0clb0jjpjl74 :

• End Unmaintained Runner Support: Support for runners tied to Java 8
  (such as Samza) will end when Beam discontinues Java 8 support [1]. Other
  unmaintained runners are deferred unless they create a major maintenance
  burden.

Twister2 runner PostCommit has been red for a while. Twister2 project also doesn't have activity for 6 years. It's time to remove the support

Need to be in Beam 2.78.0 as the current RC validation workflow included Twister2 tests

[Abacn]

R: @Amar3tto cc: @jrmccluskey (upcoming release managers)

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.82%. Comparing base (2213e54) to head (e622e41).
⚠️ Report is 48 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #40869       +/-   ##
===========================================
+ Coverage        0   79.82%   +79.82%     
===========================================
  Files           0      494      +494     
  Lines           0    83573    +83573     
===========================================
+ Hits            0    66708    +66708     
- Misses          0    16865    +16865     

Flag	Coverage Δ
python	79.82% <ø> (?)

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