Set MSRV to 1.63

[eagr]

The feature gating is already complicated enough, this could simplify the codebase a bit.

Rationale:

• legacy projects are unlikely to use latches, and new projects wouldn't base on Rust<1.63
• prominent projects that people rely on for concurrent programming (like tokio) have msrv>=1.63

So in practice, it doesn't make much sense to support Rust<1.63.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6eeec89) 100.00% compared to head (6a6487f) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #19   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          596       596           
  Branches       596       596           
=========================================
  Hits           596       596           

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