[SORMAS-Foundation#3730] build java 11 only

Author: JonasCir

.github/workflows/ci.yml

@@ -3,6 +3,9 @@
 
 name: Java CI with Maven
 
+env:
+  java: 11
+
 on:
   push:
     branches: [ development, master ]
@@ -11,29 +14,25 @@ on:
 
 jobs:
   build:
-    name: Build (Java ${{ matrix.java }})
+    name: Build (Java ${{ env.java }})
     runs-on: ubuntu-latest
 
-    strategy:
-      matrix:
-        java: [ 8, 11 ]
-
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Set up JDK ${{ matrix.java }}
+      - name: Set up JDK ${{ env.java }}
         uses: actions/setup-java@v1
         with:
-          java-version: ${{ matrix.java }}
+          java-version: ${{ env.java }}
 
       - name: Cache Maven packages
         # FIXME(@JonasCir) #3733 remove '**/*.pom' once serverlib pom is renamed
         uses: actions/cache@v2
         with:
           path: ~/.m2
-          key: ${{ runner.os }}-java-${{ matrix.java }}-m2-${{ hashFiles('**/pom.xml', '**/*.pom') }}
-          restore-keys: ${{ runner.os }}-java-${{ matrix.java }}-m2
+          key: ${{ runner.os }}-java-${{ env.java }}-m2-${{ hashFiles('**/pom.xml', '**/*.pom') }}
+          restore-keys: ${{ runner.os }}-java-${{ env.java }}-m2
 
       - name: Build with Maven
         working-directory: ./sormas-base