Bump gradle-core from 3.0 to 6.0 in /html4j-maven-plugin

Bumps gradle-core from 3.0 to 6.0.

---
updated-dependencies:
- dependency-name: org.gradle:gradle-core
  dependency-type: direct:production
...

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Author: dependabot[bot]

html4j-maven-plugin/pom.xml

@@ -122,7 +122,7 @@
       <dependency>
           <groupId>org.gradle</groupId>
           <artifactId>gradle-core</artifactId>
-          <version>3.0</version>
+          <version>6.0</version>
           <scope>provided</scope>
       </dependency>
       <dependency>