Change in process for ansible-test platform updates

[mattclay]

Starting with the ansible-core 2.18 development cycle, we're simplifying the process we use to add and remove platforms and versions to the devel version of ansible-test:

• Changes will no longer be announced here in advance. The changes will still receive changelog entries. The currently available list of platforms and versions can be found in the ansible-test completion files.
• Removal of old versions should occur at the same time new versions are added. Previously, new versions were added at least two weeks before the version they were replacing would be removed.

If a predictable schedule is required for potentially breaking changes, collection maintainers can switch from testing against the devel branch to the milestone branch.

[felixfontein]

I created #70 to discuss this.