Add sharding key for scim_oauth_access_tokens

Add sharding key for scim_oauth_access_tokens

Remove sharding_key_issue_url and complete classification:

Org-level with sharding key:

• Set gitlab_schema: gitlab_main_cell
• Add sharding_key or desired_sharding_key configuration. If the configuration is known but the chosen key doesn't yet meet not-null and foreign key requirements, you can add an exception to allowed_to_be_missing_not_null or allowed_to_be_missing_foreign_key to get the pipeline passing. Please link to a follow-up issue in a code comment next to the exception.
• See db/docs/epics.yml for an example.

Cell-local (cell-specific features):

• Set gitlab_schema: gitlab_main_cell_local
• No foreign key references to/from organization tables

User-related:

• Set gitlab_schema: gitlab_main_user
• Must strictly belong to user

Cell settings:

• Set gitlab_schema: gitlab_main_cell_setting
• No foreign key references to/from organization tables

Note: gitlab_main_clusterwide has been deprecated. Consult Tenant Scale group for alternatives.

Documentation

• Schema selection guidelines
• Sharding key definition
• desired_sharding_key backfill

Add to epic &11670