gateway: bump DavidAnson/markdownlint-cli2-action from 20.0.0 to 21.0.0

Bumps [DavidAnson/markdownlint-cli2-action](https://github.com/davidanson/markdownlint-cli2-action) from 20.0.0 to 21.0.0.
- [Release notes](https://github.com/davidanson/markdownlint-cli2-action/releases)
- [Commits](DavidAnson/markdownlint-cli2-action@992badc...30a0e04)

---
updated-dependencies:
- dependency-name: DavidAnson/markdownlint-cli2-action
  dependency-version: 21.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/dummy.yml

@@ -23,7 +23,7 @@ jobs:
         if: false
       - uses: dorny/test-reporter@dc3a92680fcc15842eef52e8c4606ea7ce6bd3f3  # v2.1.1
         if: false
-      - uses: DavidAnson/markdownlint-cli2-action@992badcdf24e3b8eb7e87ff9287fe931bcb00c6e  # v20.0.0
+      - uses: DavidAnson/markdownlint-cli2-action@30a0e04f1870d58f8d717450cc6134995f993c63  # v21.0.0
         if: false
       - uses: JamesIves/github-pages-deploy-action@6c2d9db40f9296374acc17b90404b6e8864128c8  # v4.7.3
         if: false