Require Jenkins 2.346.3 or newer

[MarkEWaite]

Modernize plugin build environment

• Escape jelly content by default
• Defend against null pointer exceptions
• Use try with resources to close files consistently
• Prevent null pointer exception
• Skip redundant null check
• Use most recent parent pom and build tooling
• Add Jenkinsfile for CI

Update the plugin to require Jenkins 2.346.3 or newer and to support running with Java 8, Java 11, and Java 17.

Resolves several spotbugs issues by using null checks and by using try with resources to assure that files are closed appropriately.

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