Prep for 2.1.0 release

[chadlwilson]

Double-checks the release process and does some cleanup needed

• fixes Release 2.1.0 #498 (kind of!)
• fixes Follow up with a rake build from a maintainer  #529 (kind of!)
• intended to be merged alongside
  • Correct Bundler 2.x Gem path on war/jar load #575
    • needed to fix specs and allow integration tests to pass
  • Correct running of integration tests #574
    • needed to ensure integration tests are run
  • Run tests on Java 25 #577

This PR:

• reviews + consolidates the history/changelog to represent user-facing changes since 2.0.5 (not CI/tests)
• corrects rdoc generation which was locked to an old, broken version
  • i manually validated the generated gem and rdocs content still look correct/good
• bump version; regenerate jar to allow use of warbler from github
  • Fix war file path as argument for jetty runner #527 (review)
• adjust gemspec dependencies to reflect what works and is validated (jruby-rack < 1.2.3 definitely won't work)

Diff to review since 2.0.5 for prospective release

• v2.0.5...chadlwilson:warbler:2.1.0-release-prep

[chadlwilson]

@headius did you get stuck doing a tag & gem release after merge? Anything I can help with?

[headius]

Oh, no I was just unsure if we're ready. I'll release today.