CI modernization

[offa]

Migration of GCC / Clang builds to Docker images. This enables eg. newer compiler version without messing with the host OS.

Additionally:

• GCC 12 added
• Update to ubuntu-latest (Runner)
• Update setup-python to main (instead of a fixed version)
• Migration to CodeCov Github Action

Please note: As before there's no coverage upload on Clang, since it doesn't produce data.

[codecov[bot]]

Codecov Report

Merging #172 (a796427) into master (100dbeb) will decrease coverage by 1.85%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
- Coverage   35.88%   34.02%   -1.86%     
==========================================
  Files          53       52       -1     
  Lines        2302     2272      -30     
  Branches        0      336     +336     
==========================================
- Hits          826      773      -53     
+ Misses       1476     1419      -57     
- Partials        0       80      +80     

Impacted Files	Coverage Δ
src/main/c/sha1/sha1.cpp	69.56% <0.00%> (-5.80%)	:arrow_down:
src/main/c/StringUtil.cpp	83.78% <0.00%> (-5.41%)	:arrow_down:
src/main/c/Server.cpp	20.92% <0.00%> (-5.16%)	:arrow_down:
src/main/c/HybiPacketDecoder.cpp	73.43% <0.00%> (-4.69%)	:arrow_down:
src/main/c/Connection.cpp	17.50% <0.00%> (-2.37%)	:arrow_down:
src/main/c/util/CrackedUri.cpp	96.42% <0.00%> (-0.07%)	:arrow_down:
src/main/c/seasocks/Credentials.h	0.00% <0.00%> (ø)
src/main/c/seasocks/StrCompare.h
src/main/c/seasocks/Logger.h	6.25% <0.00%> (+0.36%)	:arrow_up:
src/main/c/seasocks/util/Json.h	87.20% <0.00%> (+1.00%)	:arrow_up:
... and 6 more

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[offa]

The version doesn't change the actual python version used in the build, but the python setup action.

According to it's list of preinstalled software Python 3 is available by default anyway, so we can consider removing the action.

I'll check this just to be safe here … :-)

[mattgodbolt]

Oh! Perfect! Please go ahead as you see fit (happy to remove the python step too, or in another pr!)