chore: disable dependabot for maven-source-plugin

Motivation:

As long as https://issues.apache.org/jira/browse/MSOURCES-143 is not fixed

Author: slandelle

.github/dependabot.yml

@@ -4,6 +4,8 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+    ignore:
+      - dependency-name: "org.apache.maven.plugins:maven-source-plugin"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: