GnuTests.yml: use ubuntu-latest runner

[oech3]

We don't need newer glibc for GnuTests yet AFAIK. Supress renovate. Closes #368 .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.77%. Comparing base (791ae26) to head (2b8ef0b).
⚠️ Report is 77 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #392      +/-   ##
==========================================
+ Coverage   96.84%   97.77%   +0.92%     
==========================================
  Files          11       15       +4     
  Lines        1492     1751     +259     
  Branches       29       34       +5     
==========================================
+ Hits         1445     1712     +267     
+ Misses         46       38       -8     
  Partials        1        1              

Flag	Coverage Δ
macos_latest	97.77% <ø> (+0.92%)	⬆️
ubuntu_latest	97.77% <ø> (+0.92%)	⬆️
windows_latest	0.00% <ø> (ø)

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[codspeed]

Merging this PR will not alter performance

✅ 7 untouched benchmarks

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_{Comparing oech3:patch-1 (2b8ef0b) with main (c263fb1)}