limit windows builds to Rust 1.77 (to keep Windows 7 support)

[davidhewitt]

Change Summary

Rust 1.78 targets Windows 10. Python 3.9 requires Windows 10.

Until we drop Python 3.8 support, we should build for Windows on Rust 1.77.

Related issue number

Fixes #1418

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1463 will not alter performance

_{Comparing dh/windows-1.77 (ad6478e) with main (bfccf09)}

Summary

✅ 155 untouched benchmarks

[sydney-runkle]

Awesome, thanks!