ci: clean up Codecov ignore list

[marten-seemann]

Using generics allowed to remove a number of generated files.

[codecov]

Codecov Report

Merging #4081 (4a50d01) into master (d6ac630) will increase coverage by 0.30%.
Report is 4 commits behind head on master.
The diff coverage is 66.67%.

❗ Current head 4a50d01 differs from pull request most recent head 4b89ca3. Consider uploading reports for the commit 4b89ca3 to get more accurate results

@@            Coverage Diff             @@
##           master    #4081      +/-   ##
==========================================
+ Coverage   83.49%   83.79%   +0.30%     
==========================================
  Files         148      144       -4     
  Lines       15220    15110     -110     
==========================================
- Hits        12707    12661      -46     
+ Misses       2010     1948      -62     
+ Partials      503      501       -2     

Files Changed	Coverage Δ
internal/qtls/go121.go	56.84% <11.11%> (+1.74%)	⬆️
internal/handshake/crypto_setup.go	57.73% <73.33%> (+0.55%)	⬆️
internal/handshake/session_ticket.go	100.00% <100.00%> (ø)

... and 8 files with indirect coverage changes