Remove Servlet 3.0 Support in CacheControlHeadersWriter

[jzheaux]

Related to #6220

CacheControlHeadersWriter checks first to see if HttpServletRequest#getHeader is available.

Since Spring Framework 5.x has a baseline of Servlet Spec 3.1, this check is no longer necessary as well as any tests.

The task here is to remove the check, remove the tests, and change the class to simply call the getHeader method directly in the hasHeader method.

Hello, I submitted a PR for this. I think I followed the contributor rules correctly!