[DO_NOT_MERGE] Drop the `managed` Column from `team_conv` Table in Galley

[mdimjasevic]

DO NOT MERGE

... before Sept 2022 and/or before most deployments are on chart version >= 4.1.0. In case this is merged and deployed before 4.1.0 was deployed first, a transient period of the time it takes to roll out a deployment (in the order of minutes usually) of 'HTTP status 500 internal server errors' on client API calls to conversations can happen.

This PR is a follow-up to PR #2125 that was merged on Feb 14, 2022.

The PR also removes the managed key from JSON instances, but if needed, this can be pulled into a separate PR (it is an isolated commit in this PR). This makes it a client API breaking change.

Checklist

• [ ] Rebase on top of the latest develop. Make sure to update schemaVersion in Galley and rename services/galley/schema/src/V60_DropManagedConversations.hs accordingly.
• [x] The PR Title explains the impact of the change.
• [x] The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• [x] If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• [x] changelog.d contains the following bits of information (details):
  • [x] A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • [x] If a cassandra schema migration is backwards incompatible (see also these docs), measures to be taken by on-premise instance operators are explained.