Previously, SpringApplicationShutdownHook would always register a
shutdown hook, even if SpringApplication was configured not to
use a shutdown hook, such as in a war deployment. This could
result in a memory leak when the war was undeployed. The shutdown
hook registered by SpringApplicationShutdownHook would remain
registered, pinning the web application's class loader in memory.
This commit updates SpringApplicationShutdownHook so that it
registers a shutdown hook with the JVM lazily, upon registeration
of the first application context.
Fixes gh-27987
a4f1d32203