Revert "downgrade target to Java 1.8"

This reverts commit 01fe1f2.

Author: asakaxgit

pom.xml

@@ -35,8 +35,8 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <checkstyle.version>10.4</checkstyle.version>
     <maven.checkstyle.plugin.version>3.2.0</maven.checkstyle.plugin.version>
-    <maven.compiler.source>1.8</maven.compiler.source>
-    <maven.compiler.target>1.8</maven.compiler.target>
+    <maven.compiler.source>11</maven.compiler.source>
+    <maven.compiler.target>11</maven.compiler.target>
     <junit.version>5.9.1</junit.version>
   </properties>