issues bigint batch 1: swap columns and drop temporary objects

Batch 1 converts the six single-column FK columns on issues (closed_by_id, duplicated_to_id, last_edited_by_id, moved_to_id, promoted_to_epic_id, updated_by_id) to bigint by swapping the prepared columns and dropping the temporary objects.

Tasks

• Merge !255904 (merged) (swap 6 columns, 6 index swaps, 5 FK swaps, then 5 per-FK drop migrations and the temp index drop)
• Confirm on GitLab.com that the 6 columns are bigint and no bigint_idx_* or *_tmp objects remain

Notes

Index and FK preparation already merged in 19.4 (!251347 (merged), !253447 (merged), !253725 (merged)).