scripted-diff: Replace strCommand with msg_type

Summary:

-BEGIN VERIFY SCRIPT-
sed -i 's/\<strCommand\>/msg_type/g' ./src/net_processing.cpp
./src/test/fuzz/process_message.cpp
-END VERIFY SCRIPT-

Backport of core PR18533.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8460