Upgrade remainder java compiler versions to 1.8 (#4286)

Co-authored-by: Nkezhenxu94 <kezhenxu94@163.com>

apm-application-toolkit/pom.xml

@@ -27,7 +27,7 @@
     <packaging>pom</packaging>
 
     <properties>
-        <compiler.version>1.6</compiler.version>
+        <compiler.version>1.8</compiler.version>
     </properties>
 
     <modules>

apm-commons/pom.xml

@@ -37,6 +37,6 @@
 
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <compiler.version>1.6</compiler.version>
+        <compiler.version>1.8</compiler.version>
     </properties>
 </project>

test/plugin/scenarios/jdk-threading-scenario/pom.xml

@@ -26,7 +26,7 @@
 
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <compiler.version>1.6</compiler.version>
+        <compiler.version>1.8</compiler.version>
         <spring.boot.version>2.1.6.RELEASE</spring.boot.version>
     </properties>