Bump minimum tested Perl version, to match MIME::Types.

Our dependency tree includes `MIME::Types`, which in it's v2.29 release
updated it's own baseline Perl requirement to v5.16 (from back in 2012).

Thus, we need to bump our own minimum tested version to match.

Author: bleargh45

.github/workflows/perl-ci.yml

@@ -70,9 +70,6 @@ jobs:
           - "5.22"
           - "5.20"
           - "5.18"
-          - "5.16"
-          - "5.14"
-          - "5.12"
 
     container:
       image: perldocker/perl-tester:${{ matrix.perl-version }}