Update becheran/mlc action to v0.21.0 (#347)

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

Author: renovate[bot]

.github/workflows/build.yml

@@ -76,6 +76,6 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - name: 🔗 Markup Link Checker (mlc)
-      uses: becheran/mlc@v0.19.2
+      uses: becheran/mlc@v0.21.0
       with:
         args: --do-not-warn-for-redirect-to https://learn.microsoft.com*,https://dotnet.microsoft.com/*,https://dev.azure.com/*,https://app.codecov.io/* -p docfx