Fix the fuzzer for FormatVersion and FormatUserAgent

Summary:
The FormatSubVersion function has been removed in D9628, update the
fuzzer accordingly.

Test Plan:

ninja bitcoin-fuzzer
./src/test/fuzz/integer <path_to_corpus>
./src/test/fuzz/string <path_to_corpus>

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9710