Fix Java 8 API compatibility

pom.xml

@@ -49,8 +49,6 @@
         <git.commit.id>Unknown</git.commit.id>
 
         <java.version>8</java.version>
-        <maven.compiler.source>${java.version}</maven.compiler.source>
-        <maven.compiler.target>${java.version}</maven.compiler.target>
 
         <floodgate.version>2.2.0-SNAPSHOT</floodgate.version>
         <geyser.version>2.0.0-SNAPSHOT</geyser.version>
@@ -152,6 +150,8 @@
                     </execution>
                 </executions>
             </plugin>
+
+            <!-- Require newer versions for Junit5 support -->
             <plugin>
                 <artifactId>maven-surefire-plugin</artifactId>
                 <version>2.22.2</version>
@@ -160,6 +160,19 @@
                 <artifactId>maven-failsafe-plugin</artifactId>
                 <version>2.22.2</version>
             </plugin>
+
+            <!-- Verify Java 8 compatibility while compiling with a newer toolchain 
+            (i.e. check for unavailable methods) -->
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>3.10.1</version>
+                <configuration>
+                    <source>${java.version}</source>
+                    <target>${java.version}</target>
+                    <release>${java.version}</release>
+                </configuration>
+            </plugin>
         </plugins>
 
         <resources>