Logging: migrate pre-existing logs to debug logging with slf4j

[zhumin8]

There are some pre-existing logs in this repo. We should migrate them to the debug logging with slf4j introduced in https://github.com/googleapis/google-auth-library-java/pull/1586

Some extra considerations: for DefaultCredentialsProvider fine/debug level logs, perhaps consider to keep both. Also considering adding more logs for request/response that's used in the ADC flow like pingComputeEngineMetadata. context: https://github.com/googleapis/google-auth-library-java/pull/1586#discussion_r1966148365

Note, there is a parallel ticket in sdk-platform-java for migration. These are better worked together.