chore(build): downgrade POM Checker plugin to v1.7.0

To maintain compatibility with GitHub Actions that run Maven 3.8.8 by
default, we need to downgrade to a plugin version before 1.8.0.

See also: kordamp/pomchecker@8b8a94e
See also: actions/runner-images#10715

pom.xml

@@ -121,7 +121,7 @@
         <maven-resources-plugin.version>3.3.1</maven-resources-plugin.version>
         <maven-enforcer-plugin.version>3.5.0</maven-enforcer-plugin.version>
 
-        <pomchecker.version>1.11.0</pomchecker.version>
+        <pomchecker.version>1.7.0</pomchecker.version>
         <nexus-staging-plugin.version>1.7.0</nexus-staging-plugin.version>
         <checkstyle-maven-plugin.version>3.5.0</checkstyle-maven-plugin.version>
         <checkstyle.version>10.18.2</checkstyle.version>