SpringBoot Clarify that spring-boot-rsocket does not require spring-web

With the modularization spring-boot-rsocket got an implementation dependency to spring-web. This looks strange considering that there are only two usages (one that was added very recently) and they are guarded by classpath checks.

However, we do recognize that spring-web is useful to most applications. Without it, the RouteMatcher is a simple implementation that isn't optimized. The plan is therefore to add a dependency to spring-web in the starter itself.