Initial support for Servlet 6.1

[jhoeller]

Along with JPA 3.2 (#31157), the most interesting additions to Jakarta EE 11 are happening in Servlet 6.1, e.g. ByteBuffer support on ServletInputStream and ServletOutputStream:

https://deploy-preview-652--jakartaee-specifications.netlify.app/specifications/servlet/6.1/

[jhoeller]

It turns out that the built-in Servlet ByteBuffer support is not actually useful for us since we override it for Tomcat already and do not use the Servlet-based variant at all for Undertow and soon also for Jetty 12 (#32035). We could start using it reflectively when present in ServletHttpHandlerAdapter itself but that's not actually in use in its raw form on any common server, so we might rather upgrade that part once we have a Jakarta EE 11 baseline and therefore do not need reflective invocations anymore.

That said, there are a couple of new overloaded methods in Servlet 6.1, e.g. for sendRedirect. Supporting those in our mocks is straightforward, so I'll roll that part into 6.2 M1 right away.