Fix log timestamps fixed format
What does this MR do?
Ensures that the timestamp header fixed length is derived from RFC3339 UTC.
Why was this MR needed?
The timestamper package was inserting nil bytes to make the header fixed on non-UTC systems.
What's the best way to test this MR?
What are the relevant issue numbers?
Relates to !4591 (comment 1859022259)
Closes #37479 (closed)
Edited by Arran Walker