Modernize to latest versions supported by Java 8

[sghill-rewrite]

Hi!

This PR aims to modernize tooling and move this plugin closer to the recommended Jenkins baseline version without making the change too big. If this goes well, I can follow up with a subsequent PR to modernize further.

This PR was generated with an OpenRewrite recipe on Moderne.io.

index.jelly is a required file with the newer tooling.

I decided to remove the properties for jenkins.bom.artifactId and jenkins.bom.version because I thought it would simplify maintenance. SourceGraph showed this as the only repository that defines these properties.

Testing done

Ran mvn clean verify.

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[sghill]

@orrc would you be able to look at this modernization PR or recommend someone who could?

[sghill]

@jenkinsci/golang-plugin-developers are you able to take a look at this modernization PR? There are some other nice things we could do as well to reduce the burden for maintainers - like CD releases that happen entirely from GitHub Actions.