Update baseline

[timja]

This allows:

• usage of bom
• testing of something closer representing what users are running
• dropping of implied dependencies (for this and every plugin depending on mailer...)
• parent pom 4.x, -P quick-build ❤️

See guidance: https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/, (note can't use .1 as one of the test dependencies uses the .5 version)

Could probably remove some unneeded pipeline deps too but not sure if needed

cc @alecharp

requires #100