chore: replace cargo-machete with cargo-shear, removed unused deps

[CathalMullan]

Changes

Replaced cargo-machete with cargo-shear for detecting unused dependencies.

cargo-shear uses syn to parse Rust code to find unused deps, rather than relying on regex like cargo-machete. It also has some additional features, like parsing doc blocks for imports, and running against expanded code (requiring nightly). Tends to give fewer false positives as a result. It is slower to run, though.

Had to bump taiki-e/install-action in order to get a newer version of cargo-shear, which has a needed fix around doc block parsing.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: CathalMullan / name: Cathal (8107dd8)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.8%. Comparing base (95af815) to head (8107dd8).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3247   +/-   ##
=====================================
  Coverage   80.8%   80.8%           
=====================================
  Files        129     129           
  Lines      23199   23199           
=====================================
+ Hits       18745   18749    +4     
+ Misses      4454    4450    -4     

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