ci: drop Ruby 3.1 from test matrix (EOL since March 2025)

Nokogiri 1.19.x requires Ruby >= 3.2, and Ruby 3.1 reached end of life
on 2025-03-31. Remove it from the CI matrix.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zkoppert

.github/workflows/ci.yml

@@ -14,7 +14,6 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - "3.1"
           - "3.2"
           - "3.3"
           - "3.4"