Make chat_names table migration idempotent

What does this MR do and why?

This backports !141703 (merged) to 16-6-stable-ee.

GitLab 16.5.6, 16.6.4, and 16.7.2 added a migration that added columns chat_names.encrypted_token and chat_names.encrypted_token_iv. Unfortunately, the migration was added as different filenames. As a result, if users upgrade to 16.5.6 or 16.6.4 first, they'll already have those columns, and the next upgrade to 16.7.2 will fail.

To prevent an upgrade failure, we need to make the migration idempotent so that it can be applied again without issues.

For reference, these are the filenames that are used in different versions:

• 16.8 - db/migrate/20231123160255_add_token_to_chat_names.rb
• 16.7 - db/migrate/20231219120134_add_token_to_chat_names.rb
• 16.6 - db/migrate/20231215135014_add_token_to_chat_names.rb
• 16.5 - db/migrate/20231215145632_add_token_to_chat_names.rb

Relates to omnibus-gitlab#8371 (closed)

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