Bump DavidAnson/markdownlint-cli2-action action to v21 (#575)

* Bump DavidAnson/markdownlint-cli2-action action to v21

| datasource  | package                             | from    | to      |
| ----------- | ----------------------------------- | ------- | ------- |
| github-tags | DavidAnson/markdownlint-cli2-action | v20.0.0 | v21.0.0 |


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

* Update ISSUE_TEMPLATE.md

---------

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Martin Costello <[email protected]>

Author: renovate[bot]

.github/ISSUE_TEMPLATE.md

@@ -2,14 +2,14 @@
 title: ''
 ---
 
-### Expected behaviour
+## Expected behaviour
 
 <!-- Explain what you expected to happen. -->
 
-### Actual behaviour
+## Actual behaviour
 
 <!-- Explain what actually happened. If an exception occurred, please include a stack trace if available. -->
 
-### Steps to reproduce
+## Steps to reproduce
 
 <!-- A concise, repeatable, example of how to illustrate the issue. -->

.github/workflows/lint.yml

@@ -55,7 +55,7 @@ jobs:
         version: ${{ env.ZIZMOR_VERSION }}
 
     - name: Lint markdown
-      uses: DavidAnson/markdownlint-cli2-action@992badcdf24e3b8eb7e87ff9287fe931bcb00c6e # v20.0.0
+      uses: DavidAnson/markdownlint-cli2-action@30a0e04f1870d58f8d717450cc6134995f993c63 # v21.0.0
       with:
         config: '.markdownlint.json'
         globs: |