Change remove_after in what requires downtime docs

What does this MR do?

A minor update to the what requires downtime docs on column removal. Changing the remove_after date to indicate when it is safe to remove the columns.

@gl-database folks, care to take a look and give feedback on this? It started from a discussion here !48839 (comment 458331597)

Relates to: #290244 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• Database guides
• [-] Separation of EE specific content