Add support for server.server-header property

Add a `server.server-header` property which can be used to override the
`server` header usually sent back automatically by Tomcat/Jetty or
Undertow.

See https://www.owasp.org/index.php/Securing_tomcat for background.

Fixes gh-4461
Closes gh-4504

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -153,6 +153,7 @@ content into your application; rather pick only the properties that you need.
 	server.jsp-servlet.init-parameters.*= # Init parameters used to configure the JSP servlet
 	server.jsp-servlet.registered=true # Whether or not the JSP servlet is registered
 	server.port=8080 # Server HTTP port.
+	server.server-header= # The value sent in the server response header
 	server.servlet-path=/ # Path of the main dispatcher servlet.
 	server.session.cookie.comment= # Comment for the session cookie.
 	server.session.cookie.domain= # Domain for the session cookie.