feat(#949): add warning message

pom.xml

@@ -136,10 +136,16 @@ SOFTWARE.
       <version>12.5</version>
       <scope>runtime</scope>
     </dependency>
+    <!--
+      Warning!
+      We use the 4.0.0-alpha-7 version of maven-plugin-api intentionally.
+      Recent versions of the dependency are incompatible with Java 8.
+      So, don't change this version without a necessity.
+    -->
     <dependency>
       <groupId>org.apache.maven</groupId>
       <artifactId>maven-plugin-api</artifactId>
-      <version>4.0.0-rc-2</version>
+      <version>4.0.0-alpha-7</version>
       <scope>provided</scope>
     </dependency>
     <dependency>