* Build on JDK 8 and 17 as well as 11.
* Remove JDK 8 for now.
`DefaultDateTypeAdapterTest` fails.
* Tweak javadoc to avoid warnings.
Mostly these are about using `<h3>` when the previous tag was `<h1>`,
and the like. This previous tag might be implicit (part of what javadoc
itself outputs rather than the HTML in doc comments).
Apparently JDK 11 puts method javadoc inside `<h2>` while JDK 11 puts it
inside `<h3>`. Or something like that. Anyway it doesn't appear to be
possible to use `<h3>` _or_ `<h4>` and please both.
