GH Actions/basics: revert to xmllint-problem-matcher v1

As the `korelstar/xmllint-problem-matcher` repo now has a long-running `v1` branch, this update which was included in PR 30 is no longer needed (and would necessitate more frequent updates if it would remain).

Ref:
* korelstar/xmllint-problem-matcher 7

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@v1.1
+      - uses: korelstar/xmllint-problem-matcher@v1
 
       # Validate the composer.json file.
       # @link https://getcomposer.org/doc/03-cli.md#validate