remove deprecated option application-credential-source

[kheina]

No description provided.

[louisruch]

Some minor feedback on moving the reserved deprecated fields, other than that:

• You should just confirm there are no enterprise ssh specific tests that will fail when Application credentials are removed
• Add a changelog that this was removed

[kheina]

Some minor feedback on moving the reserved deprecated fields, other than that:

* You should just confirm there are no enterprise `ssh` specific tests that will fail when `Application credentials are removed`

* Add a changelog that this was removed

I didn't check ent, so I'll look there to make sure. I did find a scenario in local env and a tf doc that had it. iirc everywhere else already had it listed as deprecated. I'll double check to make sure the docs say it's been removed if it only says deprecated

[louisruch]

This is great thanks!