Plan: Remove support for Ubuntu 16.04 (Xenial)

[amanda11]

Once Ubuntu 20.04 support has been added (#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:

• st2
  • CI/CD - Add focal and remove xenial st2#5283
  • to be determined
  • identify and close all the U16 associated issues
• st2web - amanda11
  • CI/CD
• st2chatops - ~@amanda11
  • CI/CD
  • deb scriptlets
  • Dockerfile
• st2-packages - @amanda11 - Remove xenial st2-packages#704
  • remove from packages
  • remove from bash installer
  • CI/CD
  • docker-compose
• st2packaging-dockerfiles
  • U16 Dockerfiles - @amanda11
• ansible-st2
  • remove any Xenial specific checks
  • remove from integration runs
• puppet-st2 - @rush-skills
  • CI/CD
  • OS support
• st2ci - @amanda11
  • - pre-release
  • - release
• stcd - @amanda11
• st2docs - Xenial removal and roadmap update st2docs#1074
  • remove as a supported install / upgrade OS
  • Roadmap update
• st2vagrant - Add focal inst and change default from xenial st2vagrant#48
• 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 :)