Remove Perl v5.20 from our GitHub Actions test matrix.

The Perl v5.20 image is based on Ubuntu "Jessie", which is *long past*
EOL, and which can no longer cleanly install our `.deb` dependencies in
order to build/run our test suite.

So... remove it from the test matrix.

Author: bleargh45

.github/workflows/perl-ci.yml

@@ -108,7 +108,6 @@ jobs:
           - "5.26"
           - "5.24"
           - "5.22"
-          - "5.20"
 
     container:
       image: perl:${{ matrix.perl-version }}