update codecov config

[hassec]

This PR includes 3 changes:

1. start using the new codecov uploader because the bash version is deprecated and will soon stop working see here
2. ignore the samples and unitTests folders in codecov. I don't think we should measure these, they aren't really part of the exiv2 application.
3. start using gcovr to generate a summary, which is then used by codecov. This has the benefit that the taken branches are better analyzed. See this example in my fork
   Bit of a bummer that it drops the coverage by ~7% but I guess it's more accurate. And the information that e.g. only one branch of an if() is taken is IMHO potentially useful.

[codecov]

Codecov Report

Merging #1809 (28d4bb3) into main (2488fc8) will decrease coverage by 7.69%.
The diff coverage is n/a.

❗ Current head 28d4bb3 differs from pull request most recent head cdbadcf. Consider uploading reports for the commit cdbadcf to get more accurate results

@@            Coverage Diff             @@
##             main    #1809      +/-   ##
==========================================
- Coverage   68.22%   60.52%   -7.70%     
==========================================
  Files         151       96      -55     
  Lines       20930    18843    -2087     
  Branches        0     9469    +9469     
==========================================
- Hits        14279    11405    -2874     
+ Misses       6651     5151    -1500     
- Partials        0     2287    +2287     

Impacted Files	Coverage Δ
include/exiv2/error.hpp	65.51% <0.00%> (-34.49%)	⬇️
src/utils.cpp	40.38% <0.00%> (-33.09%)	⬇️
src/enforce.hpp	72.22% <0.00%> (-27.78%)	⬇️
src/psdimage.cpp	61.48% <0.00%> (-22.82%)	⬇️
include/exiv2/types.hpp	68.57% <0.00%> (-22.34%)	⬇️
src/xmpsidecar.cpp	70.00% <0.00%> (-21.00%)	⬇️
src/pngimage.cpp	60.05% <0.00%> (-18.73%)	⬇️
src/sonymn_int.cpp	51.26% <0.00%> (-17.72%)	⬇️
src/getopt.cpp	61.53% <0.00%> (-16.90%)	⬇️
src/jpgimage.cpp	70.28% <0.00%> (-16.35%)	⬇️
... and 118 more

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