When a secret's tracked revision is removed, it cannot track a new one

[Tony Meyer]

Juju 3.4.2, microk8s.

Given an application secret and a unit that is tracking a non-latest revision, if the charm executing on that unit removes the tracked revision, then it seems to get into a broken state (with respect to the secret) that it cannot get out of, in that it can't get the secret (fine - the revision is gone), but it also can't `--refresh` to move to a newer revision either.

To reproduce:

Given this charm:

```python
class TestingSecretRevisionsCharm(ops.CharmBase):
    def __init__(self, framework: ops.Framework):
        super().__init__(framework)
        framework.observe(self.on.install, self._on_install)
        framework.observe(self.on.set_action, self._on_set)
        framework.observe(self.on.get_action, self._on_get)
        framework.observe(self.on.remove_action, self._on_remove)

    def _on_install(self, event):
        self.app.add_secret({"foo": "bar"}, label="my-secret")

    def _on_set(self, event):
        secret = self.model.get_secret(label="my-secret")
        secret.set_content(event.params)

    def _on_get(self, event):
        secret = self.model.get_secret(label="my-secret")
        event.set_results(secret.get_content(refresh=event.params.get("refresh", False)))

    def _on_remove(self, event):
        secret = self.model.get_secret(label="my-secret")
        secret.remove_revision(event.params["revision"])
```

Pack and deploy, and then run:

```
juju run testing-secret-revisions/0 set two=two
juju run testing-secret-revisions/0 remove revision=1
juju run testing-secret-revisions/0 get # this will fail as expected
juju run testing-secret-revisions/0 get refresh=true # this would ideally succeed
```