Add sharding keys for incident_management

Add sharding keys for feature category incident_management.

These tables have been identified as cell local tables.

All cell local tables require a sharding key.

A sharding key has been automatically selected for these tables. The sharding key was chosen because it is a NOT NULL column referencing either projects or namespaces.

Additionally, gitlab_schema has been set to gitlab_main_cell for any tables that didn't use this schema already. For these tables we have also added allow_cross_joins, allow_cross_transactions and allow_cross_foreign_keys. These will silence any existing violations, allowing the pipeline to pass without requiring further changes. In the future, we'll remove these allow_... statements and fix any violations as they arise. You can read more about this in the documentation for multiple databases.

Reviewer: Please confirm that each table should indeed be cell local, and that the selected column is an appropriate sharding key. When you are finished, please request a review from the database maintainer suggested by Danger.

If you have any questions or concerns, reach out to @tigerwnz or @DylanGriffith.

If you would like to go through similar merged MRs so as to gather an understanding on this topic, you can use this link.

This change was generated by gitlab-housekeeper using the Keeps::DetermineShardingKeyFeatureCategory keep.

To provide feedback on your experience with gitlab-housekeeper please comment in #442003.