The annotated XHTML 1.0 renderer produces invalid attributes for metadata

The `data-*`-attributes that are used in the annotated XHTML renderer and XHTML 1.0 parser to represent metadata values are not valid in XHTML 1.0, they are a feature of HTML 5, see, e.g. this StackOverflow question. This has been introduced in XRENDERING-527.

Note that this is basically irrelevant for browsers, as browsers are just rendering HTML without any version. However, as XWiki explicitly distinguishes between HTML5 and XHTML 1.0, this is still a bug. Further, this might become relevant when explicit whitelists for attributes are used as suggested in XCOMMONS-1680, at least if this whitelist should follow the specification and metadata is needed in restricted mode. Another issues is that if XRENDERING-86 should be implemented, the metadata attributes will be lost, too.