ICSHWI-9965: Simplify protocol file names to ease macro usage

Using simpler file names like SERIAL.proto and LAN.proto allows us to easily select different names based only on a macro specifying the interface type.

Note: This MR is part of a big stack of MRs which will eventually be merged together. As one merge request depends on the previous one to be effective, please do not merge after the review.

The logical order of review for the MRs is !13 (merged) -> !18 (merged)

The merge order is the reverse: !18 (merged) -> !13 (merged)