Previously, Log4j2's own shutdown hook was only disabled when Log4j2
detected javax.servlet.Servlet on the classpath and, therefore,
determined that it was running in a web application. In an application
without Servlet on the classpath, this could lead to both Log4j2's shut
down hook and and logging system's shutdown handler both stopping
Log4j2. This could result in a failure as the second attempt at stopping
would result in reinitialization which would fail as the JVM is already
shutting down.

This commit introduces a new Log4j2 PropertySource implementation,
registered via META-INF/services, that sets the
log4j.shutdownHookEnabled property to false. This will ensure that
Log4j2's own shutdown hook is disabled by default whenever Spring Boot
is on the classpath and not just in Servlet-based web applications.

Fixes gh-26953