rollback MSRV to 1.91.x

[eshattow]

Let's lock the rust version used to not-newer-than rust v1.91.x that ships with the most recent stable Alpine Linux v3.23 release

Resolves: #1571

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.89%. Comparing base (8980d8c) to head (e479e36).
⚠️ Report is 11 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1572      +/-   ##
==========================================
- Coverage   94.92%   94.89%   -0.03%     
==========================================
  Files         156      156              
  Lines        6105     6109       +4     
  Branches      350      350              
==========================================
+ Hits         5795     5797       +2     
- Misses        248      250       +2     
  Partials       62       62              

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[codspeed-hq]

Merging this PR will not alter performance

✅ 113 untouched benchmarks

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_{Comparing eshattow:rust-version-1911r1 (e479e36) with dev (8980d8c)}