CompatHelper: bump compat for Makie to 0.21, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Makie package from 0.15.2, 0.16, 0.17, 0.18, 0.19, 0.20 to 0.15.2, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

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Fixes #65.

[coveralls]

Pull Request Test Coverage Report for Build 9292848009

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 24.286%

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Totals
Change from base Build 9292670431:	0.0%
Covered Lines:	17
Relevant Lines:	70

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💛 - Coveralls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.28%. Comparing base (1fd112e) to head (ef08d68).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   24.28%   24.28%           
=======================================
  Files           4        4           
  Lines          70       70           
=======================================
  Hits           17       17           
  Misses         53       53           

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