Bump minimum tested version of Perl to 5.10.

Our dependency tree includes `HTML::Tagset`, which now has a minimum
required version of Perl v5.10.

Bump our minimum tested version to match.

Author: bleargh45

.github/workflows/perl-ci.yml

@@ -74,7 +74,6 @@ jobs:
           - "5.14"
           - "5.12"
           - "5.10"
-          - "5.8"
 
     container:
       image: perldocker/perl-tester:${{ matrix.perl-version }}