GitHub Actions: Use quiet xmllint output to only print error lines

Success lines are no longer printed, making the CI log shorter
and faster to read.

.github/workflows/static_checks.yml

@@ -56,7 +56,7 @@ jobs:
 
       - name: Class reference schema checks
         run: |
-          xmllint --noout --schema doc/class.xsd doc/classes/*.xml modules/*/doc_classes/*.xml platform/*/doc_classes/*.xml
+          xmllint --quiet --noout --schema doc/class.xsd doc/classes/*.xml modules/*/doc_classes/*.xml platform/*/doc_classes/*.xml
 
       - name: Run C compiler on `gdextension_interface.h`
         run: |