Use "restore" more consistently in the git panel (#25780)

This PR replaces almost all uses of "discard" in the git panel UI with
"restore", since that's the verb we settled on for the project diff.

The only exception is in the confirmation prompt for restoring files,
where I've kept the "discard changes" language. I think consistency is
less important here and it's helpful to rephrase the action that's being
taken to emphasize that it's destructive.

Release Notes:

- N/A