Choose a minimum codecov patch coverage ratio

[schalkms]

Chooses a minimum patch coverage ratio that the commit must meet to be considered a success.
This applies only to PRs.

Docs:
https://docs.codecov.io/docs/commit-status

[schalkms]

Some PRs have been failing because the patch coverage only slightly dropped compared with our current ratio, which is ~83 %.
I think 80 % is a good value for the patch test coverage of a PR.

[codecov-io]

Codecov Report

Merging #2425 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2425   +/-   ##
=========================================
  Coverage     82.87%   82.87%           
  Complexity     2157     2157           
=========================================
  Files           354      354           
  Lines          6136     6136           
  Branches       1122     1122           
=========================================
  Hits           5085     5085           
  Misses          475      475           
  Partials        576      576

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