send-email: update documentation of required Perl modules

send-email: update documentation of required Perl modules

Improve and complete the list of required email related Perl modules,
clarifying which are core Perl modules and remove Net::SMTP::SSL.

git-send-email uses the TLS support in the Net::SMTP core module from
recent versions of Perl. Documenting the minimum version is complex
because of separate numbering for Perl (5.21.5~169), Net:SMTP (2.34)
and libnet (3.01). Version numbers from commit:
bfbfc9a953 ("send-email: Net::SMTP::starttls was introduced in v2.34",
2017-05-31).

Users of older Perl versions without Net::SMTP::SSL installed will get a
clear error message.

Signed-off-by: Chris Mayo <aklhfex@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>