push.yml: test Java SE 21, macOS 13/15, and Windows Server 2025

Author: stephengold

.github/workflows/push.yml

@@ -30,7 +30,7 @@ jobs:
 
   Java17-MacOS:
     if: contains(toJson(github.event.commits), '[ci skip] ') == false
-    runs-on: macOS-14
+    runs-on: macOS-13
     steps:
       - uses: actions/setup-java@v4
         with:
@@ -39,9 +39,20 @@ jobs:
       - uses: actions/checkout@v4
       - run: ./gradlew build javadoc --console=plain --stacktrace
 
+  Java21-MacOS:
+    if: contains(toJson(github.event.commits), '[ci skip] ') == false
+    runs-on: macOS-15
+    steps:
+      - uses: actions/setup-java@v4
+        with:
+          distribution: 'zulu'
+          java-version: 21
+      - uses: actions/checkout@v4
+      - run: ./gradlew build javadoc --console=plain --stacktrace
+
   Java24-Windows:
     if: contains(toJson(github.event.commits), '[ci skip] ') == false
-    runs-on: windows-2022
+    runs-on: windows-2025
     steps:
       - uses: actions/setup-java@v4
         with: