How should I modify my maintainer to use PGP?

Just update your maintainer object to contain the line:

auth: PGPKEY-XXXXXXXX

where XXXXXXXX is your key-ID.

Be aware that if there are other auth: lines in your object, all will be effective. So, if there are both auth: NONE and auth: PGPKEY-XXXXXXXX lines in the mntner object, still everybody can update it, without the need for the PGP key.