gotp enroll --fingerprints doesn't support gpg2

[gopilal]

Based on comments in => https://superuser.com/questions/1037401/pubring-gpg-and-secring-gpg-are-missing-after-key-generation

gp2 supports pubring.kbx and not pubring.gpg. should modify the publicKeyring to honor latest gpg versions.

var publicKeyring = gpgHome + "/.gnupg/pubring.gpg"

[tschuy]

Unfortunately, the library I'm using doesn't support gpg2 keystores. I've added an error message when a GPGv2 kbx store is found that at least will show that it's on my radar: https://github.com/tschuy/gotp/blob/master/cmd/root.go#L41