ci: fix deprecation 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 fix many CI warnings.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@7c7eae1). Click here to learn what that means.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #12   +/-   ##
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  Coverage        ?   39.04%           
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  Files           ?        9           
  Lines           ?     1465           
  Branches        ?      195           
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  Hits            ?      572           
  Misses          ?      799           
  Partials        ?       94           

Flag	Coverage Δ
macos_latest	37.71% <0.00%> (?)
ubuntu_latest	39.07% <0.00%> (?)

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