[CI] Remove redundant version of Julia from matrix

[giordano]

I'm using this PR also to test that the package works with GTK 3.24.29, but will remove that change afterwards

[giordano]

Ok, good news, all tests on GitHub Actions are successful with GTK 3.24.29. I'm going to remove the installation of GTK3_jll#main

[codecov]

Codecov Report

Merging #571 (1fb4f46) into master (543a4f1) will decrease coverage by 1.51%.
The diff coverage is n/a.

❗ Current head 1fb4f46 differs from pull request most recent head ca3f8f6. Consider uploading reports for the commit ca3f8f6 to get more accurate results

@@            Coverage Diff             @@
##           master     #571      +/-   ##
==========================================
- Coverage   50.27%   48.76%   -1.52%     
==========================================
  Files          32       32              
  Lines        2544     2543       -1     
==========================================
- Hits         1279     1240      -39     
- Misses       1265     1303      +38     

Impacted Files	Coverage Δ
src/precompile.jl	0.00% <0.00%> (-97.23%)	⬇️
src/Gtk.jl	88.46% <0.00%> (-7.70%)	⬇️
src/GLib/signals.jl	75.10% <0.00%> (-0.85%)	⬇️
src/GLib/gvalues.jl	86.52% <0.00%> (+0.60%)	⬆️

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