build: update action runtime to Node.js 24 (#176)

GitHub Actions is deprecating Node.js 20, with Node 24 becoming the
default runner on June 2, 2026. This updates the action to run on
the Node 24 runtime.

Changes:
- action.yml: node20 → node24
- esbuild target: node20 → node24
- @types/node: ^20 → ^24
- @actions/* deps bumped to their Node 24-compatible versions
  (highest CJS-compatible releases, avoiding the ESM-only latest)

The @actions/* bumps also resolve the punycode deprecation warning
(DEP0040) that Node 24 triggers with the older dependency chain.

Closes #172

Co-authored-by: Adam White <adam0white@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

action.yml

@@ -57,7 +57,7 @@ outputs:
     description: If the version of Bun was cached.
 
 runs:
-  using: "node20"
+  using: "node24"
   main: "dist/setup/index.js"
   post: "dist/cache-save/index.js"
   post-if: success()