Deprecate the org.springframework.web.servlet.view.document package

[bclozel]

As seen in #35432, our iText/OpenPDF and Apache POI view support is very outdated; the template method abstraction style is itself outdated in our support.

These days we would expect libraries to provide a *View class implementation, or applications to perform the rendering in web handlers. With this issue, we should deprecate types in the org.springframework.web.servlet.view.document package and update the documentation accordingly. Applications relying on existing Abstract* classes in this package can copy/paste the code and compiling them against a recent version of their library of choice.

We are scheduling removal for Spring Framework 7.1 or 7.2.

[jhoeller]

Let's consider deprecating org.springframework.web.servlet.view.feed right along since that package also just has abstract view classes against an external dependency (ROME).