ci: fix warnings by removing actions-rs/cargo and actions-rs/toolchain

[cakebaker]

This PR removes the unmaintained actions-rs/toolchain and actions-rs/cargo. actions-rs/toolchain is replaced by dtolnay/rust-toolchain and actions-rs/cargo by direct invocations of cargo. It should get rid of all most CI warnings.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9bb0ae8) 47.87% compared to head (7631e14) 47.87%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #30   +/-   ##
=======================================
  Coverage   47.87%   47.87%           
=======================================
  Files           4        4           
  Lines         470      470           
  Branches       79       79           
=======================================
  Hits          225      225           
  Misses        208      208           
  Partials       37       37           

Flag	Coverage Δ
macos_latest	46.80% <ø> (ø)
ubuntu_latest	46.80% <ø> (ø)
windows_latest	1.70% <ø> (ø)

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