diff-highlight: drop perl version dependency back to 5.8

diff-highlight: drop perl version dependency back to 5.8

The diff-highlight code does not rely on any perl features beyond what
perl 5.8 provides. We bumped it to v5.26 along with the rest of the
project's perl scripts in 702d8c1f3b (Require Perl 5.26.0, 2024-10-23).

There's some value in just having a uniform baseline for the project,
but I think diff-highlight is special here:

  - it's in a contrib/ directory that is not frequently touched, so
    there is little risk of Git developers getting annoyed that modern
    perl features are not available

  - it provides a module used by other projects. In particular,
    diff-so-fancy relies on DiffHighlight.pm but does not otherwise
    require a perl version more modern than 5.8.

Let's drop back to the more conservative requirement.

Signed-off-by: Scott Baker <scott@perturb.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>