chore: adds branch 3.3.x as stable

[thiagotnunes]

Also removes clirr / linkage monitor from non master branches since they only work for the master branch.

[codecov]

Codecov Report

Merging #1123 (5c4ff6c) into master (641f04c) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1123      +/-   ##
============================================
+ Coverage     84.97%   84.99%   +0.02%     
- Complexity     2727     2728       +1     
============================================
  Files           156      156              
  Lines         14416    14416              
  Branches       1381     1381              
============================================
+ Hits          12250    12253       +3     
- Misses         1595     1596       +1     
+ Partials        571      567       -4     

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/spanner/connection/SpannerPool.java	87.16% <0.00%> (-0.54%)	33.00% <0.00%> (ø%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	17.00% <0.00%> (-1.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.01% <0.00%> (+0.19%)	74.00% <0.00%> (ø%)
...ud/spanner/SessionPoolAsyncTransactionManager.java	86.61% <0.00%> (+1.57%)	15.00% <0.00%> (+2.00%)

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