Relax the google-api-core dependency.

[tvalentyn]

Relax the google-api-core dependency with an understanding that the incompatibility will be addressed in 2.8.3 or following next version (cc: @parthea)

This will reduce the diamond dependency risks in apache-beam in the future, when other libraries require newer versions of google-api-core.

We will also work on sunsetting the python spanner io in favor of the cross-language implementation: #22480. We will try to complete this before EOY (cc: @johnjcasey);

R: @kileys

cc: @BjornPrime

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[codecov]

Codecov Report

Merging #22513 (44902a1) into master (f4f1de3) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #22513   +/-   ##
=======================================
  Coverage   74.17%   74.17%           
=======================================
  Files         706      706           
  Lines       93226    93229    +3     
=======================================
+ Hits        69148    69152    +4     
+ Misses      22810    22809    -1     
  Partials     1268     1268           

Flag	Coverage Δ
python	83.54% <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/transforms/combiners.py	93.05% <0.00%> (-0.39%)	⬇️
...beam/testing/load_tests/load_test_metrics_utils.py	34.07% <0.00%> (-0.39%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.42% <0.00%> (-0.13%)	⬇️
setup.py	0.00% <0.00%> (ø)
...s/python/apache_beam/runners/portability/stager.py	85.54% <0.00%> (ø)
...on/apache_beam/examples/complete/juliaset/setup.py	0.00% <0.00%> (ø)
...ks/python/apache_beam/runners/worker/sdk_worker.py	88.94% <0.00%> (+0.31%)	⬆️
sdks/python/apache_beam/runners/direct/executor.py	97.01% <0.00%> (+0.54%)	⬆️
...python/apache_beam/runners/worker/worker_status.py	79.71% <0.00%> (+2.17%)	⬆️

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