ci: quote since-perl value so YAML doesn't drop trailing zero

`since-perl: 5.10` was being parsed by YAML as the float 5.1, so the
perl-actions/perl-versions matrix expansion produced "every version
since 5.1" — which included Perl 5.8 (the version after 5.6, before
5.10).

That's why CI on PR #61 ran a Perl 5.8 job and failed on `//` (the
defined-or operator added in 5.10), despite the project declaring
MIN_PERL_VERSION 5.010 in Makefile.PL and CLAUDE.md.

Quoting the value forces YAML to keep it as the string "5.10", and the
action filters correctly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: toddr

.github/workflows/testsuite.yml

@@ -63,7 +63,9 @@ jobs:
       - id: action
         uses: perl-actions/perl-versions@v1
         with:
-          since-perl: 5.10
+          # Quoted so YAML doesn't parse 5.10 as the float 5.1 (which would
+          # include Perl 5.8). Project declares MIN_PERL_VERSION 5.010.
+          since-perl: "5.10"
           with-devel: true
 
   linux: