Previously, the upgrade to Flyway 6 broke compatibility with Flyway
5.2 due to the use of method references that refer to two methods
that do not exist in Flyway 5.2.

This commit replaces the method references with lambdas that are only
called if the user sets the related property. Unless a new-in-Flyway-6
property is set, the auto-configuration will work as before. When such
a property is set the auto-configuration will fail with a
NoSuchMethodError. This approach was chosen to make the
incompatibility clear.

We have also introduced support for passing any JavaMigration beans
in the context into Flyway. This too relies on API that is new in
Flyway 6. It is possible (although unlikely) that users had
JavaMigration beans in Spring Boot 2.1 that were being ignored. This
commit restores this behaviour when using Flyway 5.2.

Closes gh-18193