fix: use ops.main() instead of ops.main.main()

Ops changed to prefer importing just `ops` and having all of the
functionality as an attribute of that top level package, rather than
needing to know what module has what functionality. I'm only changing
this because we're actually getting a lot of Deprecation warnings in our
logs.

```
unit-controller-1: 09:38:39 WARNING
unit.controller/1.leader-settings-changed
/var/lib/juju/agents/unit-controller-1/charm/./src/charm.py:162: DeprecationWarning: Calling `ops.main.main()` is deprecated, call `ops.main()` instead
unit-controller-1: 09:38:39 WARNING
unit.controller/1.leader-settings-changed   main(JujuControllerCharm)
unit-controller-1: 09:38:39 INFO juju.worker.uniter.operation ran "leader-settings-changed" hook (via hook dispatching script: dispatch)
unit-controller-1: 09:38:39 WARNING unit.controller/1.config-changed
/var/lib/juju/agents/unit-controller-1/charm/./src/charm.py:162: DeprecationWarning: Calling `ops.main.main()` is deprecated, call `ops.main()` instead
unit-controller-1: 09:38:39 WARNING unit.controller/1.config-changed main(JujuControllerCharm)
```

However, I went through and updated the other places as well.

src/charm.py

@@ -7,19 +7,15 @@
 import secrets
 import urllib.parse
 import yaml
+import ops
 from charms.prometheus_k8s.v0.prometheus_scrape import MetricsEndpointProvider
-from ops.charm import CharmBase
-from ops.framework import StoredState
-from ops.charm import RelationJoinedEvent, RelationDepartedEvent
-from ops.main import main
-from ops.model import ActiveStatus, BlockedStatus, Relation
 from typing import List
 
 logger = logging.getLogger(__name__)
 
 
-class JujuControllerCharm(CharmBase):
-    _stored = StoredState()
+class JujuControllerCharm(ops.CharmBase):
+    _stored = ops.StoredState()
 
     def __init__(self, *args):
         super().__init__(*args)
@@ -38,7 +34,7 @@ def __init__(self, *args):
             self.on.metrics_endpoint_relation_broken, self._on_metrics_endpoint_relation_broken)
 
     def _on_start(self, _):
-        self.unit.status = ActiveStatus()
+        self.unit.status = ops.ActiveStatus()
 
     def _on_config_changed(self, _):
         controller_url = self.config["controller-url"]
@@ -61,7 +57,7 @@ def _on_website_relation_joined(self, event):
         port = self.api_port()
         if port is None:
             logger.error("machine does not appear to be a controller")
-            self.unit.status = BlockedStatus('machine does not appear to be a controller')
+            self.unit.status = ops.BlockedStatus('machine does not appear to be a controller')
             return
 
         address = None
@@ -75,7 +71,7 @@ def _on_website_relation_joined(self, event):
                     'port': str(port)
                 })
 
-    def _on_metrics_endpoint_relation_created(self, event: RelationJoinedEvent):
+    def _on_metrics_endpoint_relation_created(self, event: ops.RelationJoinedEvent):
         username = metrics_username(event.relation)
         password = generate_password()
         self.control_socket.add_metrics_user(username, password)
@@ -84,7 +80,7 @@ def _on_metrics_endpoint_relation_created(self, event: RelationJoinedEvent):
         try:
             api_port = self.api_port()
         except AgentConfException as e:
-            self.unit.status = BlockedStatus(
+            self.unit.status = ops.BlockedStatus(
                 f"can't read controller API port from agent.conf: {e}")
             return
 
@@ -110,7 +106,7 @@ def _on_metrics_endpoint_relation_created(self, event: RelationJoinedEvent):
         )
         metrics_endpoint.set_scrape_job_spec()
 
-    def _on_metrics_endpoint_relation_broken(self, event: RelationDepartedEvent):
+    def _on_metrics_endpoint_relation_broken(self, event: ops.RelationDepartedEvent):
         username = metrics_username(event.relation)
         self.control_socket.remove_metrics_user(username)
 
@@ -141,7 +137,7 @@ def ca_cert(self) -> str:
         return self._agent_conf('cacert')
 
 
-def metrics_username(relation: Relation) -> str:
+def metrics_username(relation: ops.Relation) -> str:
     """
     Return the username used to access the metrics endpoint, for the given
     relation. This username has the form
@@ -159,4 +155,4 @@ class AgentConfException(Exception):
 
 
 if __name__ == "__main__":
-    main(JujuControllerCharm)
+    ops.main(JujuControllerCharm)