Update to rand v0.10

[Expyron]

• Update rand to v0.10
• Replace rand_chacha with chacha20
• Various mechanical fixes due to a few small breaking changes (rename Rng to RngExt, ChaChaRng to ChaCha20Rng)
• Increase MSRV to 1.85 to be compatible

[barrbrain]

README.md has a stale reference to the MSRV and we should update that too. Bumping the MSRV is worth a mention in the commit message or as a separate commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.02%. Comparing base (47fbf8a) to head (e3484be).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3477   +/-   ##
=======================================
  Coverage   82.02%   82.02%           
=======================================
  Files          84       84           
  Lines       23838    23838           
=======================================
  Hits        19553    19553           
  Misses       4285     4285           

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[Expyron]

I split the change into two commits for clarity.