Fix poms, modernize

[daniel-beck]

Motivation

The plugins in this repository stand out as being by far the least actively maintained among the top 100 plugins by popularity.

They're by far the least recently released (March 2016) and have by far the oldest core dependency (1.580.1). Since ace-editor is a dependency of workflow-cps, there's no modern Pipeline-based Jenkins possible without it, making removal of unused implied dependencies cumbersome to impossible.

PR Content

This fixes the pom references broken since 2048280, and updates to the newest plugin parent POM.

As a result, this now depends on Jenkins 2.361 and requires Java 11 to build.

Also documented the core.symlinks requirement for Git.

Testing

Did some minimal testing of ace-editor via workflow-cps. While the generated JS output is slightly different (probably from an earlier, unreleased change), basic features (error annotation, collapsing blocks, syntax highlighting) worked.

[alecharp]

Works well locally

[MarkEWaite]

Yes, many times yes. Modernizing ace-editor will avoid future pain if jdk-tool or one of the older split plugins is deprecated. The deprecation of windows-slaves highlighted the type of problems that users can see due to implied dependencies.

[basil]

jenkinsci/workflow-cps-plugin#653 is likely more maintainable.

[daniel-beck]

@basil This is great, thank you.