chore(librarian): Add forward slash to remove regex (#14657)

This PR adds a forward slash to the `remove_regex` to provide clarity
that the regex is for the full directory name not partial name.
For example, for `google-cloud-compute` , where there also exists
`google-cloud-compute-v1beta`

We would have

```
    remove_regex:
      - packages/google-cloud-compute/
```

which would only match `google-cloud-compute`  

instead of

```
    remove_regex:
      - packages/google-cloud-compute
```

Which would match both `google-cloud-compute` and
`google-cloud-compute-v1beta`

Author: parthea