fix: add rust-version to Cargo.toml

[xtqqczze]

verified with cargo hack check --workspace --all-targets --feature-powerset --rust-version

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.93%. Comparing base (d19ce8b) to head (80520b3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #808   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files          35       35           
  Lines        7251     7251           
  Branches      378      378           
=======================================
  Hits         6666     6666           
  Misses        443      443           
  Partials      142      142           

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[github-actions]

Commit 80520b3 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 266 / FAILED: 41 / SKIPPED: 6

Changes from main branch:
  TOTAL: +1
  PASSED: +1
  FAILED: +0

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing xtqqczze:msrv (80520b3) with main (d19ce8b)}

[sylvestre]

still draft ?