`gh repo rename` can only rename a repo, but not change its owner
(i.e. transfer it).
As flagged in #10034, at the moment, the CLI behaves like it can do
this, and produces weird results.
If I ask to rename my repo to `polyseam/frappe-containers`:
```bash
gh repo rename polyseam/frappe-containers
```
..the preview suggests that it will do the right thing:
```
? Rename polyseam/containers to polyseam/frappe-containers? Yes
```
...but the repo gets renamed wrongly:
```
✓ Renamed repository polyseam/polyseam-frappe-containers
```
This adds an upfront validation that looks for a slash and tells
you that the command can't transfer a repo.
Fixes#10034.
