Improve build trace scroll controls

What does this MR do?

The trace step buttons now correctly scroll to the top or bottom of the build trace, rather than jumping inconsistently and never full reaching the top due to the fixed position of the header/navigation area.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Build trace UX

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Contributes to #21832 (closed).

Added 13 commits:

• 9e0c447a...14174375 - 11 commits from branch master
• b5d74bef - trace step buttons now scroll 50px up or down at a time
• f162dde6 - Now anchors to the top or bottom of the build trace with the correct offset

Reassigned to @jschatz1

Milestone changed to %8.12

Added ~149423 label

Status changed to merged

Mentioned in commit 3be079c2

Picked into 8-12-stable, will go into RC6.

Removed ~149423 label

Mentioned in commit e5e2f0b5

Mentioned in issue #23092 (moved)