Change configuration key in Java version matrix

In the Github workflow configuration, the Java version matrix's key was incorrectly set as "java". It has now been corrected to "version". Additionally, the string interpretation is removed from 'matrix.version' in the 'java-version' field to simplify the code syntax.

.github/workflows/main.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: [ self-hosted, small ]
     strategy:
       matrix:
-        java:
+        version:
           - 6
           - 7
           - 8
@@ -33,7 +33,7 @@ jobs:
         uses: actions/setup-java@v2
         with:
           distribution: 'zulu'
-          java-version: "${{ matrix.version }}"
+          java-version: '${{ matrix.version }}'
 
       - name: Install Dependencies
         run: gradle resolveDependencies