SpringBoot Set default values for showSummary and showSummaryOutput

This PR changes to set default values for showSummary and showSummaryOutput in LiquibaseProperties to align with its configuration metadata.

See gh-38522

Comment From: wilkinsona

The null values are intentional. Making this change will break compatibility with earlier versions of Liquibase and cause Liquibase423AutoConfigurationTests to fail, effectively reverting #38522.

Comment From: izeye

@wilkinsona Ah, sorry, I missed the point, but I thought default values in configuration metadata are for explicit default values in configuration properties, not for implicit default values. Am I misunderstanding it?

Comment From: wilkinsona

They can be used for implicit default values, too, as is the case here. https://github.com/spring-projects/spring-boot/commit/3e4e59a8f03bc5ca5d7cad90e0e58e8bcf6a277d updated the tests to ensure that the metadata and the implicit defaults are in sync. That said, this is a somewhat unusual situation as we only have this arrangement for backwards compatibility.