chore(deps): update dependency markdownlint-cli2 to v0.21.0 (#39)

| datasource | package           | from   | to     |
| ---------- | ----------------- | ------ | ------ |
| npm        | markdownlint-cli2 | 0.20.0 | 0.21.0 |

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package-lock.json

@@ -62,7 +62,7 @@
     "@types/node": "24.11.0",
     "@vitest/ui": "4.0.18",
     "eslint": "9.39.3",
-    "markdownlint-cli2": "0.20.0",
+    "markdownlint-cli2": "0.21.0",
     "prettier": "3.8.1",
     "typescript": "5.9.3",
     "typescript-eslint": "8.56.1",