remove goreportcard badge as it is discontinued

[6543]

see https://goreportcard.com/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (b7b09a3) to head (4d2ba1c).
⚠️ Report is 53 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #132      +/-   ##
========================================
- Coverage    4.28%      0   -4.29%     
========================================
  Files           8      0       -8     
  Lines         514      0     -514     
========================================
- Hits           22      0      -22     
+ Misses        491      0     -491     
+ Partials        1      0       -1     

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