Prior to this commit, the audit auto-configuration provided
an `InMemoryAuditEventRepository` bean. This commit changes the auto-config
so that an `AuditEventRepository` is not provided and instead the auto-config
is conditional on the presence of a `AuditEventRepository` bean. This is done
to encourage the use of a custom implementation of `AuditEventRepository`
since the in-memory one is quite limited and not suitable for production.
A flag is available if the auto-configuration needs to be turned off even
in the presence of a bean.

Closes gh-16110