Use release for fmt and mimalloc when multiple available, remove erroneous mimalloc 2 forcing

[timkpaine]

• Libraries may allow multiple available (as with vcpkg) in the form Release;Debug, so take the first if this is the case
• The mimalloc min version part accidentally forces mimalloc 2, where 3 is widely available now

xref: microsoft/vcpkg#50116

[MikePopoloski]

mimalloc v3 is not fully production ready yet (the project says the current release is "rc3") which is why I hadn't updated yet. It's not generally safe to just take a different major version the project hasn't tested against, though for the case of mimalloc it's likely that things will build just fine.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (1b521a3) to head (47fde9f).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1680   +/-   ##
=======================================
  Coverage   94.53%   94.53%           
=======================================
  Files         235      235           
  Lines       53711    53711           
=======================================
  Hits        50776    50776           
  Misses       2935     2935           

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[timkpaine]

Just sharing for visibility, I'm going to patch for vcpkg and conda anyway