chore: drop unused deps and correct engines.node floor

[finalerock44]

Remove four dependencies that nothing in the action references:

• node-fetch, @types/node-fetch, @octokit/rest, form-data

src/ imports only @actions/core and @actions/github, and none of the four appear as a require/import in the built bundle. form-data's 18 textual hits in dist are MIME strings and comments inside undici's own multipart code, not the package; its only dependent was @types/node-fetch, so removing that orphaned it. Rebuilding after removal produces a byte-identical dist/index.js, confirming all four were dead weight. 247 lines drop out of the lockfile, shrinking the surface that generates Dependabot alerts.

Also raise engines.node from >=20.0.0 to >=22.19.0. undici 8.x requires

=22.19.0 and is the highest floor in the tree, so the old value was already
inconsistent before this change. The action itself runs on action.yml's node24 runtime; this only constrains local dev and CI.

Pre-existing and untouched: tsc --noEmit reports TS2304 'Cannot find name ErrorOptions' in @octokit/request-error's .d.ts, because tsconfig sets lib: ["es2016"] and ErrorOptions is ES2022. Reproduces identically on main. It does not affect pnpm build or pnpm test.

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