Move CHANGELOG entries of !5361, !5451 and !5887 from 8.11 to 8.12

What does this MR do?

Move CHANGELOG entries of !5361 (merged), !5451 (merged) and !5887 (merged) from 8.11 to 8.12

Why was this MR needed?

They were wrongly added to 8.11 here:

• https://gitlab.com/gitlab-org/gitlab-ce/commit/220755f52ad6e3fdfa43c62e0a4a4051721246dc#note_14900001
• https://gitlab.com/gitlab-org/gitlab-ce/commit/87f93ceb9542c415d6c23c72ff93d63272107d54