CompatHelper: bump compat for "Polynomials" to "0.7"

[github-actions]

This pull request changes the compat entry for the Polynomials package from 0.5.2, 0.6 to 0.5.2, 0.6, 0.7.

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 #76 into master will decrease coverage by 5.56%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   51.56%   46.00%   -5.57%     
==========================================
  Files           6        6              
  Lines         223      250      +27     
==========================================
  Hits          115      115              
- Misses        108      135      +27     

Impacted Files	Coverage Δ
src/Collections.jl	45.05% <0.00%> (-6.20%)	⬇️
src/LinearFitting.jl	30.76% <0.00%> (-4.02%)	⬇️
src/Find.jl	0.00% <0.00%> (ø)

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