Raise minimum Perl to 5.8

[JRaspass]

ATM This dist claims to support Perl 5.6, however:

• We don't test on 5.6, 5.8 is the lowest GitHub Actions goes.
• Many of our dependencies only support 5.8+ like HTML::Form, HTML::HeadParser, HTTP::Cookie, etc. So in practice it's impossible to install a modern WWW::Mechanize on 5.6.
• CPANTS doesn't even go back that far, so clearly no one is using 5.6 - http://matrix.cpantesters.org/?dist=WWW-Mechanize+2.14

Additionally we can do some slight code cleanup as our test suite currently does this in a bunch of files:

delete @ENV{qw( PATH IFS CDPATH ENV BASH_ENV )};  # Placates taint-unsafe Cwd.pm in 5.6.1