Modernize plugin development

[MarkEWaite]

Modernize plugin development

Allow build with Java 11. Simplify dependency management with the plugin bill of materials. Allow incremental builds to be deployed for easier testing.

• Use parent pom 4.31
• Use read only URL for SCM connection
• Use GitHub for plugin documentation
• Increase spotbugs effort to detect issues
• Check dependencies with dependabot
• Allow incremental builds to be deployed
• Remove a Guava usage from test and a deprecation
• Simplify dependency management with plugin bom
• Use antisamy-markup-formatter 2.4
• Add CONTRIBUTING file
• Add 'Please report an issue' link to README

[MarkEWaite]

Ugh, started from an outdated base branch. Closing this pull request