Consider removal of GUIDs

We have now introduced custom slugs for monitor lookups (https://github.com/getsentry/sentry/issues/45166). We are currently using the monitor_id parameter in the route as way to lookup GUIDs or slugs. All existing monitors have had their guids migrated to the slugs column. We should consider if we want to completely drop guids. There is one primary thing holding us back from this: * It is difficult to use slugs in the API endpoint that does not have organization context ``` POST sentry.io/api/0/monitors/:monitorGuid/checkins/ ``` Since there is no organization in the URL we would NEED to rely on the authorization method to provide the organization. This would NOT work with session based authorization (theoretically possible if we remember the current org). But would work OK with DNS and organization token level authentication, since those have a direct association to the organization. ```[tasklist] - [ ] Remove the GUID column (continue generating slugs as GUIDs). This includes removing the dual writing, however we will still generate a GUID for all new monitors as the slug. - [ ] Update the UI to allow defining slugs when creating monitors. We can be clever here and generate it as they type the name of the monitor, and then let them edit it if they would like. ```