CI: Add jruby-9.4.12.1

This is the new target version for openvoxserver/db 8:
https://github.com/OpenVoxProject/jruby-deps/blob/a50c718fee1a3df385959cf02dd4b61503b53472/project.clj#L1

Author: bastelfreak

.github/workflows/ci.yaml

@@ -39,6 +39,7 @@ jobs:
         ruby: ${{ fromJSON(needs.rubocop_and_matrix.outputs.ruby) }}
         include:
           - ruby: 'jruby-9.4.8.0'
+          - ruby: 'jruby-9.4.12.1'
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout current PR
@@ -139,6 +140,7 @@ jobs:
           - {os: ubuntu-22.04, ruby: '3.2'} # with openssl 3
           - {os: ubuntu-22.04, ruby: 'jruby-9.3.14.0'}
           - {os: ubuntu-latest, ruby: 'jruby-9.4.8.0'}
+          - {os: ubuntu-latest, ruby: 'jruby-9.4.12.1'}
           - {os: windows-2022, ruby: '2.7'}
           - {os: windows-2022, ruby: '3.2'} # with openssl 3
     runs-on: ${{ matrix.cfg.os }}