exp gc: deprecate in 3.0

[dberenbaum]

dvc exp gc has become redundant with dvc exp remove. It's confusing because:

1. Both commands do the same thing in different ways.
2. It doesn't actually gc the DVC-tracked data, so it's name and relation to dvc gc is not obvious.
3. gc syntax is harder to understand. This is justified for the cache where we want to ensure we don't delete something referenced in multiple places, but each exp ref is unique, so I don't see the same need here.

[dberenbaum]

Related to https://github.com/iterative/dvc/issues/7093.

[dberenbaum]

Closing as a duplicate of #7093