build(deps): bump reviewdog/action-shellcheck from 1.18.1 to 1.21.0

Bumps [reviewdog/action-shellcheck](https://github.com/reviewdog/action-shellcheck) from 1.18.1 to 1.21.0.
- [Release notes](https://github.com/reviewdog/action-shellcheck/releases)
- [Commits](reviewdog/action-shellcheck@50e5e1e...6e3a862)

---
updated-dependencies:
- dependency-name: reviewdog/action-shellcheck
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/shellcheck.yml

@@ -22,7 +22,7 @@ jobs:
       - name: shellcheck
         # Make sure the action is pinned to a commit, as all reviewdog repos
         # have hundreds of contributors with write access (breaks easy/often)
-        uses: reviewdog/action-shellcheck@50e5e1e2284575f23a1e409d9c0804cdfc4f6e31  # v1.18.1
+        uses: reviewdog/action-shellcheck@6e3a862f231c6895fbd335b70adef8f9243d5762  # v1.21.0
         with:
           filter_mode: "file"
           fail_on_error: true