fix CI matrix: compute java version inline instead of matrix include

The include entry created a new matrix combination without the server
dimension, causing an empty SERVER_VARIANT and a spurious job. Use a
startsWith expression to select Java 25 for 26.x, 21 for everything else.

Author: def9a2a4

.github/workflows/server-test.yml

@@ -13,21 +13,17 @@ jobs:
         minecraft: ['1.21.1', '1.21.4', '1.21.8', '1.21.10', '1.21.11', '26.1.2']
         # minecraft: ['1.21.1', '1.21.11']
         server: ['paper', 'purpur']
-        java: ['21']
-        include:
-          - minecraft: '26.1.2'
-            java: '25'
 
     name: ${{ matrix.server }} ${{ matrix.minecraft }}
 
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Java ${{ matrix.java }}
+      - name: Set up Java
         uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
-          java-version: '${{ matrix.java }}'
+          java-version: ${{ startsWith(matrix.minecraft, '26.') && '25' || '21' }}
 
       - name: Set up Node.js
         uses: actions/setup-node@v4