Intent to Deprecate CGI, and move to support mod_perl via PSGI

[Dylan Hardison]

I intend to deprecate CGI -- which means the following:

1) We need a solid, well documented migration path from CGI to PSGI.
This is of a prime concern to our Windows users (who largely rely on
CGI) but other platforms as well.
Note that the performance difference for CGI vs. persistent perl
interpreter on Windows will be enormous because of how slow
CreateProcess() is on Windows.

2) We won't remove CGI on a whim -- deprecating it means that we will
not be beholden to the requirements of CGI. Especially when we can
sacrifice compile-time speed for runtime speed.

Further, supporting anything other than PSGI is a burden we need not bear.
We will continue to support mod_perl, but we do not need to keep
maintaining the code in mod_perl.pl. We can replace that code with a
smaller footprint of https://metacpan.org/pod/Plack::Handler::Apache2
which is already in our dependency list. [Bug 1241149]

(As an aside, we can support CGI in a similar way with
Plack::Handler::CGI, but not without
changing the urls (http://bugzilla.example/bugzilla.cgi/index.cgi)
or URL rewriting, and a significant performance penalty.)

[Bug 1241149] https://bugzilla.mozilla.org/show_bug.cgi?id=1241149