CompatHelper: bump compat for "JuMP" to "0.21"

[github-actions]

This pull request changes the compat entry for the JuMP package from 0.20.0 to 0.20.0, 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.

[codecov]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files           1        1           
  Lines          95       95           
=======================================
  Hits           91       91           
  Misses          4        4           

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