Detect compiler version from gcc output if not specified.

[Spacetown]

This PR selects the latest GCC version on the system for the tests. If only gcc is available the version is detected from the output of gcc --version.

If using qa_compiler session the default is switched from gcc-5 to gcc-11 (the current latest version for the reference data).

[codecov]

Codecov Report

Base: 95.22% // Head: 95.22% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f059af0) compared to base (28e5c88).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #686   +/-   ##
=======================================
  Coverage   95.22%   95.22%           
=======================================
  Files          24       24           
  Lines        3495     3498    +3     
  Branches      615      615           
=======================================
+ Hits         3328     3331    +3     
- Misses         94       95    +1     
+ Partials       73       72    -1     

Flag	Coverage Δ
ubuntu-18.04	?
ubuntu-20.04	94.13% <ø> (?)
windows-2019	94.90% <ø> (+0.12%)	⬆️

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Impacted Files	Coverage Δ
gcovr/tests/test_gcovr.py	97.72% <ø> (ø)
gcovr/gcov.py	74.54% <0.00%> (-1.46%)	⬇️
gcovr/writer/html.py	95.29% <0.00%> (ø)
gcovr/utils.py	91.21% <0.00%> (+2.11%)	⬆️

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[latk]

Thank you for working on this! A couple of minor comments though.

Do the changes to the compiler selection interact with the set_environment() function?

FYI, the Github Actions logs are full of deprecation warnings, we should eventually address those. This is actually partially related to the GCC version problem since the Actions rely on some really ancient Ubuntu versions in order to install some compilers.

[Spacetown]

The following warnigng is not resolvable yet since PR seanmiddleditch/gha-setup-ninja#15 is still open.

Node.js 12 actions are deprecated. For more information see: 
https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. 
Please update the following actions to use Node.js 16: seanmiddleditch/gha-setup-ninja

[Spacetown]

@latk Most warnings are fixed but the following warning I don't understand:

The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files.
For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

We don't use set-output we only use the environment files linked in the information linked above. In this documentation following syntax is used:

echo "VARIABLE=MyValue" >> $GITHUB_ENV

But we use a different quoting since a long time:

echo VARIABLE="MyValue" >> $GITHUB_ENV

With the quoting from the documentation it doesn't work. The environment isn't set.