Removes unsupported Python 3.6 from the release validation script

[chamikaramj]

This causes release validation to take an unnecessarily long time and may cause release validation to fail in environments where Python 3.6 is not available.

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[chamikaramj]

R: @tvalentyn @youngoli

[codecov]

Codecov Report

Merging #17397 (8cb3e3a) into master (4b709d5) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17397      +/-   ##
==========================================
- Coverage   73.76%   73.76%   -0.01%     
==========================================
  Files         686      686              
  Lines       90143    90143              
==========================================
- Hits        66491    66490       -1     
- Misses      22482    22483       +1     
  Partials     1170     1170              

Flag	Coverage Δ
python	83.66% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/python/apache_beam/utils/interactive_utils.py	92.68% <0.00%> (-2.44%)	⬇️
...pache_beam/runners/interactive/interactive_beam.py	81.09% <0.00%> (-0.73%)	⬇️
...runners/interactive/display/pcoll_visualization.py	85.85% <0.00%> (-0.51%)	⬇️
...eam/runners/interactive/interactive_environment.py	90.21% <0.00%> (-0.31%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.64% <0.00%> (+0.12%)	⬆️
sdks/python/apache_beam/transforms/combiners.py	93.42% <0.00%> (+0.38%)	⬆️
sdks/python/apache_beam/io/localfilesystem.py	92.24% <0.00%> (+0.77%)	⬆️

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[tvalentyn]

Python 3.6 was supported up until last release.
It is being removed in #17252 which will be merged shortly.

[tvalentyn]

cc: @AnandInguva