fix(gemspec): Ignore build artifacts from package

[tvdeyen]

What is this pull request for?

Since we release the gem via github actions we
must ignore build artifacts like vendor/bundle.

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (d9112f4) to head (cd65af3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3598   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files         314      314           
  Lines        8225     8225           
=======================================
  Hits         8017     8017           
  Misses        208      208           

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[alchemycms-bot]

💚 All backports created successfully

Status	Branch	Result
✅	8.0-stable

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details