GH Actions: update the xmllint-problem-matcher

The `xmllint-problem-matcher` action runner has released a new version which updates it to use node 16.
This gets rid of a warning which was shown in the action logs.

Refs:
* https://github.com/korelstar/xmllint-problem-matcher/releases/tag/v1.1

Author: jrfnl

.github/workflows/ci.yml

@@ -41,7 +41,7 @@ jobs:
 
       # Show violations inline in the file diff.
       # @link https://github.com/marketplace/actions/xmllint-problem-matcher
-      - uses: korelstar/xmllint-problem-matcher@v1
+      - uses: korelstar/xmllint-problem-matcher@v1.1
 
       # Validate the composer.json file.
       # @link https://getcomposer.org/doc/03-cli.md#validate