Plan: Remove support for Ubuntu 16.04 (Xenial)

[amanda11]

Once Ubuntu 20.04 support has been added (https://github.com/StackStorm/discussions/issues/68), then we should remove support for Ubuntu 16.

Good first issues! Removing is easy. If someone from the community wants to help with these tasks, this is a great way to help us with the v3.5.0 release activities. Feel free to ask questions here or in our #development Slack channel.

Checklist:

• [x] st2
  • [x] CI/CD - https://github.com/StackStorm/st2/pull/5283
  • [x] to be determined
  • [x] identify and close all the U16 associated issues
• [x] st2web - amanda11
  • [x] CI/CD
• [x] st2chatops - ~@amanda11
  • [x] CI/CD
  • [x] deb scriptlets
  • [x] Dockerfile
• [x] st2-packages - @amanda11 - https://github.com/StackStorm/st2-packages/pull/704
  • [x] remove from packages
  • [x] remove from bash installer
  • [x] CI/CD
  • [x] docker-compose
• [x] st2packaging-dockerfiles
  • [x] U16 Dockerfiles - @amanda11
• [x] ansible-st2
  • [x] remove any Xenial specific checks
  • [x] remove from integration runs
• [x] puppet-st2 - @rush-skills
  • [x] CI/CD
  • [x] OS support
• [x] st2ci - @amanda11
  • [X] - pre-release
  • [x] - release
• [x] stcd - @amanda11
• [x] st2docs - https://github.com/StackStorm/st2docs/pull/1074
  • [x] remove as a supported install / upgrade OS
  • [x] Roadmap update
• [x] st2vagrant - https://github.com/StackStorm/st2vagrant/pull/48
• [x] ~st2docker - move from xenial to focal~ st2-docker is based on bionic

[winem]

I'm currently working on the support for Ubuntu 20.04 in ansible-st2 and will remove the support for 16.04 at the same time. This is already in progress and I'm close to provide the PRs.

I can also take care of the st2docs topics.

[rush-skills]

Can update puppet-st2 work completed in this issue too :)