Change `spring-boot-tomcat-runtime` and `spring-boot-jetty-runtime`
into starter POMs and reduce the number of dependencies needed for
`spring-boot-tomcat` and `spring-boot-jetty`.
The runtime starters provide only the jars required to run the
embedded server along with the module jar itself (excluding transitive
dependencies) and `spring-boot-webserver` (excluding transitive
dependencies).
The build setup required for an executable jar is slightly different
between Maven and Gradle. For Maven, the regular module is put in the
`provided` scope. For Gradle, the regular module remains in main
configuration and the runtime jar is put in the `providedRuntime`
configuration. The reference documentation has been updated to
show how to configure things if starters are being used.
Manual testing has been performed to ensure that wars build with Maven
and Gradle work with both Tomcat and Jetty in both deployed and
`java -jar` modes.
Closes gh-48175
Phillip Webb
2685f4bf29