Coverage analysis with codecov.

[hriener]

This PR integrates the code coverage analysis app codecov.

This PR also removes some GitHub flow actions for Ubuntu due to the decision by virtual-environments not to support "GCC and clang versions less than 9.x on Ubuntu" anymore: actions/runner-images#2950

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0b98f81). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 84dbe5b differs from pull request most recent head 057edc6. Consider uploading reports for the commit 057edc6 to get more accurate results

@@            Coverage Diff            @@
##             master     #434   +/-   ##
=========================================
  Coverage          ?   79.74%           
=========================================
  Files             ?      117           
  Lines             ?    12720           
  Branches          ?        0           
=========================================
  Hits              ?    10144           
  Misses            ?     2576           
  Partials          ?        0           

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[coveralls]

Pull Request Test Coverage Report for Build 722111737

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 80.086%

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Totals
Change from base Build 673727868:	0.0%
Covered Lines:	10661
Relevant Lines:	13312

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💛 - Coveralls