The `reactor-tools` dependency now brings a new Reactor Debug Agent
which instruments loaded classes for better Reactor stacktraces.
This commit removes the `spring.reactor.stacktrace-mode.enabled`
configuration property since the related Reactor Hook is about to be
removed.
As a replacement, we're introducing `spring.reactor.debug-agent.enabled`
which tells whether the Reactor Debug Agent should be loaded, given that
the `reactor-tools` dependency is available. This option is enabled by
default, since adding the dependency on classpath is a strong signal
already.

Fixes gh-17128