Document processes for out of support operating systems

[sxa]

This is a follow-on to the work in #2902

In addition to the main machines in the jenkins CI, we have systems in various other places that we use as part of the infrastructure. This issue will cover making sure we have documentation on what to do when an operating system goes out of support, so we can remove it from the system entirely where feasible, and also to cover policies surrounding new releases (Both STS and LTS) of the platforms we use and ideally timescale targets.

Relevant previous issues:

• AIX 7.1
• Ubuntu 16.04 removal
• [Document https://github.com/SSDF PW6.1: Documentation of build servers, compilers, tools, build process #2503

Places to consider:

• Jenkins / inventory (real machines)
• DockerStatic containers used for test (See relevant EPIC)
• VagrantPlaybookCheck/QemuPlaybookCheck
• DockerFileCheck (Is this still useful?)
• Github actions (e.g. this PR)

Anything I've missed?