Deprecate `uniforms-bootstrap3` package

[piotrpospiech]

Deprecate the Bootstrap3 theme, as mentioned in this issue (https://github.com/vazco/uniforms/issues/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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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