Swap merge_request_user_mentions.note_id for self-managed instances

What does this MR do and why?

This MR swaps themerge_request_user_mentions.note_id and merge_request_user_mentions.note_id_convert_to_bigint columns for self-managed instances. This was already done for GitLab.com with !117343 (merged).

The process is outlined here.

The timeline for GitLab.com was the following:

• 15.9 !111267 (merged)
• 15.11 !113928 (merged)
• 16.1 !123108 (merged)

Since we decided to commit schema changes to structure.sql for the above MRS, we now need to consider the state of the schema from which the self-managed instance was created.

We have the following cases:

• Instances created from 15.8 and prior – swap the columns
• Instances created from 15.9, 15.10 – swap the columns
• Instances created from 15.11 – do not swap as the columns are already swapped
• Instances created from 16.1 – do not swap as the columns are already swapped, and the cleanup is done (no more temp column and trigger)

Related to #417402 (closed)

How to set up and validate locally

I have done the following manual testing:

1. Create a new instance from each of 15.8.0, 15.9.0, 15.11, and 16.1.0
2. Upgrade all instances to 16.1.2
3. Execute the migrations from this MR
4. Verify migrations execute successfully and merge_request_user_mentions table has the expected schema.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #417402 (closed)