Allow DB migrations without DataSourceProperties

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

src/checkstyle/import-control.xml

@@ -4,6 +4,7 @@
 	<allow pkg=".*" regex="true" />
 
 	<subpackage name="autoconfigure">
+		<disallow class="org.springframework.boot.autoconfigure.jdbc.DataSourceProperties"/>
 		<subpackage name="web">
 			<allow pkg="org.springframework.boot.web.server" />
 			<allow pkg="org.springframework.boot.web.servlet.server" />