JDK 13+ : Headings should start with h2 and h4

[yeikel]

Headings in documentation comments for all modules, packages and types (including nested types) should begin at <h2>.
Headings in members should begin at <h4>.

Currently, this leads to the following failure:

MavenReportException: Error while generating Javadoc: 
[ERROR] Exit code: 1
[ERROR] /MyClass.java:24: error: unexpected heading used: <H1>, compared to implicit preceding heading: <H1>
[ERROR]  * <h1>My heading</h1>

Changing it to <h2> is enough to fix it

See https://bugs.openjdk.org/browse/JDK-8220379