refactor: rename command -> message type in comments in the src/net* files

Summary:
Co-authored-by: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>

This concludes backport of core#24141
https://github.com/bitcoin/bitcoin/pull/24141/commits/e71c51b27d420fbd6cc0a36f62e63e190e13473a

Depends on D18252

Test Plan: proof-read, search for lack of other occurences of "[Cc]ommand" in net.h and net.cpp

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18253