Pin once_cell@1.20.3 for older Rust versions

[arik-so]

No description provided.

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.21%. Comparing base (4c43a5b) to head (b434ec8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3659      +/-   ##
==========================================
- Coverage   89.24%   89.21%   -0.03%     
==========================================
  Files         155      155              
  Lines      119280   119280              
  Branches   119280   119280              
==========================================
- Hits       106446   106421      -25     
- Misses      10240    10255      +15     
- Partials     2594     2604      +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[TheBlueMatt]

Ugh