Update `FlywayAutoConfiguration`, `LiquibaseAutoConfiguration` and
`DataSourceInitializer` classes so that they no longer depend on
`DataSourceProperties`. DB migrations can now be performed against
a `@Bean` defined primary `DataSource` with an alternative
username/password.

This update also removed using fallback properties when a custom
connection `url` is defined with Flyway or Liquibase. We now assume
that `username`, `password` and `driver-class-name` will be provided
if the default values are unacceptable. Our previous logic was
particularly flawed if a custom URL caused a change of driver type.

Closes gh-25643