Prevent use of 'trunk' as a Stable Version

[Ipstenu]

In order to make rollbacks more stable, and reduce friction with users, the time has come.... to chop down trunk.

​https://make.wordpress.org/core/2022/06/26/rollback-feature-testing-call-to-action

Currently if you use Stable tag: trunk in your readme, it will pull the files from trunk. And that's great, but it can cause issues with figuring out what the rollback should be.

It shouldn't be bad with our proposed changes, but it does mean that when we want to expand and say "rollback 2 versions" (or even to a specific version), if everything's always in trunk and never tags... can't download the old version :(

Trunk as 'stable' is meeting the end of it's life.

The 'best' path might be to check the readme.txt for a stable tag value and, if it's trunk, error back "Stable Tag cannot be trunk. It must match the Version as indicated in your plugin headers." Bonus, make sure we only check for the FIRST iteration (some people re-used Stable Tag per version in their changelogs).

[dd32]

In 12614:

Plugin Directory: Readme Validator: Don't encourage the use of Stable Tag: trunk for plugins.

Props Ipstenu.
Fixes #6987.
See #6380.

[dd32]

In 13726:

Plugin Directory: Import: When importing from SVN, if the plugin specifies a Stable Tag that does not exist, and we fall back to trunk, warn them.

See #6380, #6108.
Fixes #5645.