Add expires_at to service_access_token table

What does this MR do and why?

Add expires_at column to the service_access_token table.

The table is currently no used an has no records.

How to set up and validate locally

Run the migration

In rails console, check that the field is visible:

[7] pry(main)> Ai::ServiceAccessToken.new
=> #<Ai::ServiceAccessToken:0x0000000280876388
 id: nil,
 created_at: nil,
 updated_at: nil,
 category: nil,
 encrypted_token: nil,
 encrypted_token_iv: nil,
 expires_at: nil,
 token: nil>

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 #417450 (closed)