refactor: replace zap with slog

[danielandersson]

This PR replaces the third-party Zap logging library with Go's standard library slog package for structured logging. This change reduced the number of dependencies while maintaining the same logging functionality and format.

Fixes #2120

Changes

• Removed dependencies on go.uber.org/zap and go.uber.org/zap/zapcore
• Added usage of Go's standard library log/slog package
• Simplified the StdLogger structure by combining infoLog and debugLog into a single stdLog
• Refactored the StructuredLogger to use slog.Logger instead of Zap's SugaredLogger
• Modified the NewStructuredLogger function to use slog handlers and configuration
• Added a new replaceAttr function to maintain compatibility with Google Cloud Logging format
• Ensured that log messages continue to adhere to the LogEntry format for Google Cloud Logging

[jackwotherspoon]

Thanks @danielandersson 👏

I will take a look at this in the coming days 😄

[jackwotherspoon]

@hessjcg mind taking a look at this one?

Also @enocom you might also want to take a look as a similar change would probably be added to AlloyDB Proxy 😄

[danielandersson]

Some minor style nits. Otherwise LGTM.

@hessjcg we should manually verify the before and after logs just to be certain this doesn't break the existing structured logging. I think it's correct, but better to double check.

Thanks for the review, all of your feedback should have been addressed with 0946cd0c922c7e0d24912087bc9469e497f42e78.

[hessjcg]

/gcbrun

[hessjcg]

/gcbrun