Deprecate uniforms-bootstrap3 package

[piotrpospiech]

Deprecate the Bootstrap3 theme, as mentioned in this issue (#1315).

• Removed the whole package from the uniforms repository
• Removed package from the labeler
• Removed Boostrap3 from the Readme file
• Removed the uniforms-bootstrap3 package from the package.json files
• Removed theme from tsconfig references

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.49%. Comparing base (1436d4c) to head (427e676).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   94.37%   94.49%   +0.11%     
==========================================
  Files         229      203      -26     
  Lines        3754     3396     -358     
  Branches     1011      918      -93     
==========================================
- Hits         3543     3209     -334     
+ Misses         81       70      -11     
+ Partials      130      117      -13     

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