Rework `org.springframework.boot.context.embedded` to relocate classes
to `org.springframework.boot.web`. Packages are now organized around
the following areas:

Packages for shared concerns, for example the `WebServer` interface
to start/stop a server and the common configuration elements:
- org.springframework.boot.web.context
- org.springframework.boot.web.server

Servlet specific packages:
- org.springframework.boot.web.servlet.server
- org.springframework.boot.web.servlet.context
- org.springframework.boot.web.servlet.filter

Reactive specific packages:
- org.springframework.boot.web.reactive.context
- org.springframework.boot.web.reactive.server

Embedded server implementations (both reactive and servlet):
- org.springframework.boot.web.embedded

In addition:

- Rename `EmbeddedServletContainerFactory` to `ServletWebServerFactory`
  to align with the `ReactiveWebServerFactory`.
- Rename `EmbeddedWebApplicationContext` to
  `ServletWebServerApplicationContext` and
- Rename `EmbeddedReactiveWebApplicationContext` to
  `ReactiveWebServerApplicationContext`.
- Add checkstyle rules to restrict imports.
- Fixup all affected code to use the correct imports and local names.

Fixes gh-8532