Remove execute_batched_migrations_on_schedule_ci_database feature flag

What does this MR do and why?

This MR removes execute_batched_migrations_on_schedule_ci_database feature flag, and replaces it with the execute_batched_migrations_on_schedule flag, as discussed in #360224 (comment 950715484).

This way we have one flag to control batched background migration execution, and after !86525 (merged) each worker will pick and process only migrations for its database.

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 #360224 (closed)