Resolve "Swap snippet_user_mentions to bigint (Self-host)"

What does this MR do and why?

This MR swaps thesnippet_user_mentions.note_id and snippet_user_mentions.note_id_convert_to_bigint columns for self-managed instances. This was already done for GitLab.com with Swap snippet_user_mentions.note_id to bigint (!115456 - merged). The process is outlined here.

The timeline for GitLab.com was the following:

• 15.9 Initialize conversion of snippet_user_mentions.... (!111253 - merged)
• 15.11 Swap snippet_user_mentions.note_id to bigint (!115456 - merged)
• 16.1 Cleanup PK conversion for few notes related tables (!123108 - merged)

Since we made the decision to commit schema changes to structure.sql for the above MRs, we now need to take into account the state of the schema from which the self-managed instance was created.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Create new instance from each of 15.8.0, 15.9.0, 16.0.0, 16.1.0 and 16.2.0
2. Upgrade all instances to 16.2.1
3. Execute the migrations from this MR
4. Verify migrations execute successfully and snippet_user_mentions table has the expected schema.

Numbered steps to set up and validate the change are strongly suggested.

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)