t/lib-gpg: introduce new prereq GPG2

t/lib-gpg: introduce new prereq GPG2

GnuPG v2.0.0 released in 2006, which according to its release notes

	https://gnupg.org/download/release_notes.html

is the "First stable version of GnuPG integrating OpenPGP and S/MIME".

Use this version or its successors for tests that will fail for
versions less than v2.0.0 because of the difference in the output on
stderr between the versions (v2.* vs v0.* or v2.* vs v1.*). Skip if
the GPG version detected is less than v2.0.0.

Do not, however, remove the existing prereq GPG yet since a lot of tests
still work with the prereq GPG (that is even with versions v0.* or v1.*)
and some systems still use these versions.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>