Update schema for scim_oauth_access_tokens

Resolves Add sharding key for `scim_oauth_access_tokens` (#553107).

What does this MR do and why?

This MR changes scim_oauth_access_tokens from gitlab_main to gitlab_main_cell_local schema to properly classify it for the Cells architecture.

This table is used for instance-level SCIM authentication on self-managed GitLab instances, not for organization-level functionality. The group-level SCIM functionality has already been separated into the group_scim_auth_access_tokens table which is properly sharded by group_id.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Verify the table schema classification is correct:

   Gitlab::Database::Dictionary.entry('scim_oauth_access_tokens').gitlab_schema
   # Should return 'gitlab_main_cell_local'

MR acceptance checklist

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