(maint) Update PR testing

Test on both main & 3.x instead of 4.x, test all supported versions of
Ruby on Linux, test JRuby 9.3 & 9.4 (support 2.6 & 3.1 compat resp), and
run oldest and newest versions of Ruby on Windows. Also update windows
to use latest release rather than 2019.

.github/workflows/rspec_tests.yml

@@ -3,7 +3,8 @@ name: Rspec tests
 on:
   pull_request:
     branches:
-      - 4.x
+      - main
+      - 3.x
 
 jobs:
   rspec_tests:
@@ -16,8 +17,9 @@ jobs:
           - {os: ubuntu-latest, ruby: 3.1}
           - {os: ubuntu-latest, ruby: 3.2}
           - {os: ubuntu-latest, ruby: jruby-9.3}
-          - {os: windows-2019, ruby: 2.6}
-          - {os: windows-2019, ruby: 2.7}
+          - {os: ubuntu-latest, ruby: jruby-9.4}
+          - {os: windows-latest, ruby: 2.6}
+          - {os: windows-latest, ruby: 3.2}
 
     runs-on: ${{ matrix.cfg.os }}
     steps: