Drop bullseye from CI, since the bullseye-security repo is no longer available

[filbranden]

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Also, to speed up things, if you could kindly "Allow edits and access to secrets by maintainers" in the
PR settings, as this allows us to rebase the PR on master, fix conflicts, run coverage and help with
implementing code and tests.

Description of the Change

Drop bullseye from CI, since the bullseye-security repo is no longer available. The CI for bullseye stopped working, so it makes sense to drop it.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.06%. Comparing base (f59b0d2) to head (b097032).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1643      +/-   ##
==========================================
- Coverage   77.37%   77.06%   -0.32%     
==========================================
  Files         165      165              
  Lines       15747    15747              
==========================================
- Hits        12185    12136      -49     
- Misses       2356     2409      +53     
+ Partials     1206     1202       -4     

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