CI: Drop old platforms

Author: nobu

.github/workflows/macos.yml

@@ -19,7 +19,7 @@ jobs:
       fail-fast: false
       matrix:
         os:
-        - macos-12
+        - macos-13
         - macos-14
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
     steps:

.github/workflows/ubuntu.yml

@@ -21,7 +21,6 @@ jobs:
         os:
         - ubuntu-24.04
         - ubuntu-22.04
-        - ubuntu-20.04
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
     steps:
     - name: git config