This commit ensures that Tomcat is the first reactive server configured
if the Tomcat dependency is on classpath.

Spring Boot chose Reactor Netty as the default for the reactive web
starter, but the Reactor Netty dependency can be used also for its HTTP
client. In case developers are adding Tomcat, Undertow or Jetty on their
classpath, we must configure those and consider Reactor Netty for the
client only.

Fixes gh-12176