Stop exempt_from_sharding from being used

We have a number of tables where we allow exempt_from_sharding to skip sharding. This should not be allowed anymore. As it breaks the isolation of Organizations.

With disallowing exempt_from_sharding, we might discover additional tables that needs to be sharded.

Action items

• Update to https://docs.gitlab.com/development/cells/#exempting-certain-tables-from-having-sharding-keys to discourage exempt_from_sharding strongly
• For current tables where exempt_from_sharding: true, review it again. Remove exempt_from_sharding: true where it is not used correctly - and open a sharding_key_issue_url issue to start the sharding process.
• Nice to have : Could we track the tables with exempt_from_sharding in https://cells-progress-tracker-gitlab-org-tenant-scale-g-f4ad96bf01d25f.gitlab.io/schema_migration ?
• Remove the exempt_from_sharding method once all instances of its use are removed: !204351 (merged)

/cc @nhxnguyen @mandrewsgl