Previous to this commit, an auto-configuration would create a
PlatformTransactionManager if none is present and others conditions are
met. Spring Framework now has the notion of TransactionManager, a parent
interface that gathers both PlatformTransactionManager and
ReactiveTransactionManager.
Spring Boot should not be in a situation where both managers are defined
out-of-the-box. This commit makes sure to back-off if any
TransactionManager is available.
Closes gh-22851
085091dbe3