Resolve Javadoc issue

Javadoc needs to be told that our source is targeting JDK 8 in order to
build in Java 11/12 without error.

Ref: https://bugs.openjdk.java.net/browse/JDK-8212233

honeybadger-java/src/main/java/io/honeybadger/reporter/dto/Backtrace.java

@@ -38,6 +38,8 @@ public Backtrace(final ConfigContext config, final Throwable error) {
 
     /**
      * For the benefit of deserialization
+     *
+     * @param config injected
      */
     @JsonCreator
     public Backtrace(@JacksonInject("config") final ConfigContext config) {

pom.xml

@@ -353,6 +353,9 @@
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-javadoc-plugin</artifactId>
                 <version>${maven-javadoc-plugin.version}</version>
+                <configuration>
+                    <source>8</source>
+                </configuration>
             </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>