[RFC Outreachy] gpg-interface: Replace strbuf_split()* with string_list_split()

Hello,

In continuation of my search for an appropriate microproject after temporarily moving on from my mail to the list which was about the check-whitespace script check for \No newline, I stumbled on the thread https://public-inbox.org/git/20250731225433.4028872-1-gitster@pobox.com/. After studying the thread and searching the codebase for other instances where strbuf_split()* can be replaced with string_list_split(), I saw some cases in gpg-interface.c where get_ssh_key_fingerprint and get_default_ssh_signing_key functions use the strbuf_split_max to split the input string and I think they can be good candidates for replacement with string_list_split().

Please, will this be good for a microproject? I will also appreciate any guides.

Thanks Bello