Use correct dependency name syntax

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#ignore
says

> For Java dependencies, the format of the dependency-name attribute
> is: groupId:artifactId (for example: org.kohsuke:github-api).

.github/dependabot.yml

@@ -13,5 +13,5 @@ updates:
     ignore:
       # maven plugin updates break plugin bill of materials
       # do not offer minor version updates
-      - dependency-name: "maven-plugin"
+      - dependency-name: "org.jenkins-ci.main:maven-plugin"
         update-types: ["version-update:semver-minor"]