Create a Dependabot equivalent for CWP plugin lists or add support of Jenkins updates to pom.xml

Currently Jenkins X Serverless does not have automatic update for Custom WAR Packager definitions. There are 2 options we could use:

• Option 1: Use pom.xml as plugin list input, it's already supported by Custom War Packager.
  • In such case we also get upper bounds dependency checks for plugins OOTB, so that the build fails on conflicting dependencies even before starting the build
  • Problem: Dependabot does not seem to scan Jenkins Maven repositories. Could it be tweaked somehow
• Option 2: Implement dependabot plugin for BOM.yml (Jenkins JEP-309)

CC jstrachan jrawlings cosmin_cojocar