This commit migrates `AnnotationConfigReactiveWebApplicationContext`
parent to the `GenericApplicationContext` abstraction. Any use of
`AnnotationConfigWebApplicationContext` is also removed as it also
inherits from the `AbstractRefreshableApplicationContext` outdated
hierarchy.

A new `AnnotationConfigServletWebApplicationContext` context is
introduced instead, extending from `GenericApplicationContext` and
providing the counter part of the reactive context for the Servlet-based
web app tests.

See gh-16096