Removing vault-owning user can lead to orphaned, unmanagable vaults

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/hub/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/hub/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

If a user is removed from Hub, but the user was the only-owner of a vault, this vault becomes orphaned. Once orphaned, it cannot be managed anymore, but nobody can claim ownership for it. Keeping it in this state.

### System Setup

```markdown
- Hub: 1.3.3
- Keycloak: 23.0.6
```

### Steps to Reproduce

1. Create user "bert" in Keycloak
2. Login with bert in Hub
3. Create vault "foo"
4. Log out
5. Remove user "bert" in Keycloak
6. Log in as an admin
7. View details of vault "foo"

### Expected Behavior

Claim ownership of vault is possible.

### Actual Behavior

Cannot claim ownerhship, even as admin. Vault cannot be managed anymore. Need to use recovery key and manually set up vault members + storage side.

### Reproducibility

Always

### Relevant Log Output

_No response_

### Anything else?

Current workaround is to recreate the vault with the recovery key, add all members again and replace the vault config on the storage side.