[github-actions] fix markdown-lint-check (openthread#11074)

Author: jwhui

.github/workflows/build.yml

@@ -72,7 +72,7 @@ jobs:
         script/make-pretty check
 
   markdown-lint-check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - name: Harden Runner
       uses: step-security/harden-runner@0080882f6c36860b6ba35c610c98ce87d4e2f26f # v2.10.2