Enable parallel execution for batched migrations by default

What does this MR do and why?

Enable by default the batched_migrations_parallel_execution feature flag. This feature flag has been enabled for GitLab.com since 2023-01-25.

Document usage and how to configure the max number of migrations executed in parallel (the application setting was introduced with Configurable parallel batched migrations limit (!114666 - merged)).

Screenshots or screen recordings

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

How to set up and validate locally

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 #372316 (closed)