borg2: Rename `rest:` URI scheme to `ssh:`

After the removal of the legacy `ssh:` RPC protocol with #9732 (with a few exceptions, see below), it might be worth considering renaming the `rest:` URI scheme of the new REST protocol to `ssh:`.

At this point, use of the legacy `ssh:` URI scheme is limited to `--from-borg1` (e.g. `borg2 transfer --from-borg1`). Since `--from-borg1` only supports the legacy RPC protocol anyway, Borg could select that mode implicitly when this option is used. The only other component that still uses the legacy RPC protocol is `borg2 serve` without `--rest`. However, `borg2 serve` is not affected by URI scheme naming, as it selects its mode via CLI options rather than a repo URI. This would free up the `ssh:` URI scheme while keeping the remaining borg1 compatibility paths available through explicit compatibility modes.

We could then rename the `rest:` URI scheme back to `ssh:`. Historically, the URI scheme indicated how the connection to the remote repo was established. From a user's perspective, this was very intuitive. With the introduction of `rest:`, that association was somewhat lost.

Prior discussion: https://github.com/borgbackup/borg/issues/9593#issuecomment-4682065257
Related: #9766