Bump DavidAnson/markdownlint-cli2-action action to v21 (#3545)

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: Piotr Kiełkowicz <[email protected]>

Author: renovate[bot]

.github/workflows/markdownlint.yml

@@ -15,7 +15,7 @@ jobs:
       uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
 
     - name: run markdownlint
-      uses: DavidAnson/markdownlint-cli2-action@992badcdf24e3b8eb7e87ff9287fe931bcb00c6e # v20.0.0
+      uses: DavidAnson/markdownlint-cli2-action@30a0e04f1870d58f8d717450cc6134995f993c63 # v21.0.0
       with:
         globs: |
           **/*.md

src/OpenTelemetry.Instrumentation.Runtime/README.md

@@ -286,7 +286,7 @@ The number of work items that are currently queued to be processed
 by the thread pool.
 
 | Units     | Instrument Type         | Value Type | Attribute Key(s) | Attribute Values |
-| --- | ----------------------- | ---------- | ---------------- | ---------------- |
+| --------- | ----------------------- | ---------- | ---------------- | ---------------- |
 | `{items}` | ObservableUpDownCounter | `Int64`    | No Attributes    | N/A              |
 
 #### process.runtime.dotnet.**timer.count**