Change MR Header "Code" button to be secondary type

Context

This issue tracks the upodate to the Code button in MRs after a discussion about it arose in #429852 (comment 1699121753)

We introduced an inconsistency with this change as we show the Code button as a primary on other pages too, like e.g. on the project overview page: http://gdk.test:3000/flightjs/Flight

Tasks

• [ ] Provide guidance on how to proceed with the button update

Further info

By breaking away this part into this issue, we essentially unblock the rest of the simplification work in #429852 (closed)