[cleanup][io] Remove Pulsar IO Flume connector

[lhotari]

Motivation

Apache Flume is not maintained anymore (mailing list vote, website notice).
Since it's not maintained, vulnerable library dependencies won't be addressed. It's better to drop Pulsar IO Flume connector going forward

Modifications

• Remove pulsar-io/flume module and references to the module

Documentation

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

Documentation update in apache/pulsar-site#1067

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.38%. Comparing base (c572e82) to head (2f101ae).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #25079      +/-   ##
============================================
+ Coverage     73.78%   74.38%   +0.60%     
+ Complexity    33904    33638     -266     
============================================
  Files          1921     1903      -18     
  Lines        150566   149737     -829     
  Branches      17498    17403      -95     
============================================
+ Hits         111088   111383     +295     
+ Misses        30552    29458    -1094     
+ Partials       8926     8896      -30     

Flag	Coverage Δ
inttests	26.20% <ø> (-0.04%)	⬇️
systests	22.98% <ø> (+0.12%)	⬆️
unittests	73.93% <ø> (+0.66%)	⬆️

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see 139 files with indirect coverage changes

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