fix: fix loading spinner responsiveness when default font size is modified

[amtins]

Description

This PR fixes the responsiveness of the loading spinner when the default font size of .video-js is modified.

loading-spinner.webm

Specific Changes proposed

• Use em instead of px to make the component responsive

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #8295 (fc2d839) into main (f1558c6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #8295   +/-   ##
=======================================
  Coverage   82.36%   82.36%           
=======================================
  Files         112      112           
  Lines        7483     7483           
  Branches     1804     1804           
=======================================
  Hits         6163     6163           
  Misses       1320     1320           

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