Backfill organization_id in scim_oauth_access_tokens

This is part of Add sharding key for `scim_oauth_access_tokens` (#553107) and a follow-up to Add `organization_id` to `scim_oauth_access_tok... (!212160 - merged).

What does this MR do and why?

This MR backfills organization_id for existing instance-level (i.e. no Gitlab.com involved) SCIM access tokens to make the table compliant with Protocells.

This migration sets organization_id to 1 (default organization) for all existing instance-level tokens that don't already have it.

Group-level tokens are not affected as they are stored in a separate table.

References

• Add sharding key for `scim_oauth_access_tokens` (#553107)
• Add `organization_id` to `scim_oauth_access_tok... (!212160 - merged)

How to set up and validate locally

1. Create some test data in rails console:

ScimOauthAccessToken.create!(group_id: nil, organization_id: nil, token_encrypted: 'test')

2. Run the migration:

bundle exec rails db:migrate

3. Verify the backfill in rails console:

ScimOauthAccessToken.where(group_id: nil).pluck(:organization_id)
# Should all be 1

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