Add deprecated annotations for XHTML

[spassarop]

No description provided.

[spassarop]

@davewichers I could add warnings on logic flows which assume XHTML is used if you think those deprecations are not enough.

The problem is that it's only one if-block. Other usages are inline and I cannot determine which value the directive has, like:

transformer.setOutputProperty(OutputKeys.METHOD, useXhtml ? "xml" : "html");

Then it may not be that useful. The other option is to always warn, which may be a lot (or not?).

[davewichers]

I think this is good enough for now.