Register logging system's shutdown hook by default

[philwebb]

The discussion in #24507 makes me think that we should consider enabling the property by default. Whilst the arguments in #4026 are still valid, they aren't really typical Spring Boot deployments.

[wilkinsona]

I think it makes sense to flip the default now.

[wilkinsona]

The changes in this branch flip the default. While working on them, it occurred to me that it'd be nice to disable it automatically in a war deployment. We already disable the application's shutdown hook in SpringBootServletInitializer and this feels similar. It's tricky to pass the decision from the initialiser into the logging application listener. Perhaps we could add a lowest precedence property source to the environment to leave it disabled by default?

[wilkinsona]

We're going to try setting something via the servlet context in the web application case to disable the shutdown hook.