Previously, Spring Boot's Gradle plugin provided custom software
components (bootJava and bootWeb) for publishing the artifacts created
by the standard bootJar and bootWar tasks respectively. Providing a
custom software component requires the usage of some of Gradle's
internal APIs and this has now proven to be more trouble than it is
worth.

This commit removes the custom software component and documents how
to publish the artifact created by a BootJar or BootWar task directly
instead.

Closes gh-9153