Add `SpringApplicationShutdownHook` to manage orderly application
shutdown, specifically around the `LoggingSystem`. `SpringApplication`
now offers a `getShutdownHandlers()` method that can be used to add
handlers that are guaranteed to only run after the `ApplicationContext`
has been closed and is inactive.

Fixes gh-26660

Fixes gh-26850

Closes gh-26846

Closes gh-26845

Closes gh-26764

* pr/26816:
  Polish "Fix Optaplanner starter url"
  Fix Optaplanner starter url

Closes gh-26816

See gh-26816

See gh-26816

Closes gh-26843

Closes gh-26841

Closes gh-26842

Closes gh-26839

Closes gh-26838

Closes gh-26837

Closes gh-26762

Closes gh-26772

Closes gh-26770

Closes gh-26765

Closes gh-26763

Closes gh-26771

See gh-26682

Closes gh-26828

Closes gh-26620