Fix destinations and plugins in docs

[bernt-matthias]

Describe the bug

In #7854 (comment) destinations have been renamed to environments (and plugins to runners). Unfortunately this change has not been done in the docs and admins. This seems cause problems for users (ie. admins).

From the PR:

Fix up some terminology (at least from a user facing standpoint) that @natefoo and I have wanted to cleanup for a while. In particular they are runners not plugins now (since everything is a plugin) and they are environments not destinations (since destination sounds like a physical location, when really you are defining a lot of context besides where the job runs - you define how it runs also and how it is configured - so environment feels more appropriate).

The motivation is fine (and we definitely should not change it back), but it would have been much better (IMO) to simply define (and document) the terminology instead of relying on the intuitive meaning of a single word (which never can grasp such a complex concept .. in particular not for everyone).

[bernt-matthias]

and we definitely should not change it back

Hmm not sure anymore.

• We even have config variables using the term destination, e.g. tool_destinations_config_file. In this config file the term environment is used in all sorts of meanings, but not the desired, destination is used all over the place.
• and in this config file we have destination and default_destination
• And of course the docs ... where environment is used only in the container docs.
• Also the community used destination

At least training material uses the term environment.

In summary it might be a good idea to revert to "destination" in https://github.com/galaxyproject/galaxy/blob/dev/lib/galaxy/jobs/__init__.py and to properly define the term in the docs.