Make Julia v1.10 (LTS) min version

[simonschoelly]

This PR makes Julia v1.10 - the latest long-term support version the minimum version of this package.

Additionally

• Remove some code that was only necessary for older versions
• Remove Compat.jl dependency as it is not needed anymore (we can always add it back)
• Add tests for squash(::AbstractGraph) in order to not decrease code coverage

I noticed there are some TODO comments for changing the error types in some cases when we upgrade Julia - I left them in for now as this might be a slightly breaking case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.41%. Comparing base (24539fd) to head (d9f049e).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #432      +/-   ##
==========================================
+ Coverage   97.30%   97.41%   +0.10%     
==========================================
  Files         117      120       +3     
  Lines        6948     6953       +5     
==========================================
+ Hits         6761     6773      +12     
+ Misses        187      180       -7     

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[Krastanov]

I am strongly in favor. A cursory look through the edits does not raise any issues for me, but I am not deeply familiar with the codebase yet. LGTM for what is worth.

[simonschoelly]

I am strongly in favor. A cursory look through the edits does not raise any issues for me, but I am not deeply familiar with the codebase yet. LGTM for what is worth.

Thanks. Most of the changes was the removal of the @compat macro - we needed that because ismutable was not available before Julia v1.7. And then these changes lowered the coverage percentage so I added a test to bring the percentage back up.