Add upgrade notes guidelines for batched background migrations

What does this MR do and why?

This change adds new documentation requirements for database migrations that run in the background. The update requires developers to write upgrade notes for significant migrations to help customers (especially those managing their own GitLab installations) better plan and prepare for upgrades.

The new guidelines specify when these notes are needed - mainly for migrations that work on large amounts of data, could impact performance, or provide configuration options. The documentation also outlines what information should be included in these notes, such as explaining what the migration does, how long it might take, what preparation steps customers should take, and any tools they can use to estimate the impact on their system.

Essentially, this makes the upgrade process more transparent and predictable for GitLab customers by providing them with better information about significant database changes before they upgrade.

References

An example of recent upgrade note: Add CI builds metadata migration configuration ... (!214376)

MR acceptance checklist

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