Plugin Directory: Advise developers to stop using trunk

[Ipstenu]

While using trunk technically works fine, it means that it's harder to detect rollbacks (if you say 'stable is trunk' then we have to scan main plugin files to determine the version and what the previous one might be).

Stable Tag should be the stable tag to minimize confusion and maximize compatibility.

[Presskopp]

"so that all plugins can rolled back" => "so that all plugins can be rolled back" ?

[yani.iliev]

The corrections include: Changed "We ask you no longer attempt" to "We ask that you no longer attempt". Changed "can rolled back" to "can be rolled back".

[dd32]

Previously / similar / something: #6380

I'm inclined that perhaps #5484 and #6086 should come before #6380 though, so this warning-only is better than an outright block.

[Ipstenu]

@dd32 That's why I thought slipping this in to change the notice (which really only will be seen by new devs) would be a good start.

[dd32]

In 12614:

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

Props Ipstenu.
Fixes #6987.
See #6380.