This commit restores auto-configuration for using an Embedded broker
with ActiveMQ classic.
Contrary to its 2.7.x version, "spring-boot-starter-activemq" no longer
adds the broker for consistency with Artemis, and to keep the existing
3.x behavior. Rather than "inMemory", a "s.a.embedded.enabled"
property has been reintroduced that matches the name used by Artemis.
The documentation has been updated to mention that the broker
dependency must be added to use it.
Closes gh-38404
Phillip Webb
Stéphane Nicoll