logger: ensure compatibility with new log/slog package

[kevinburkesegment]

The API for the logger changes only slightly and should continue to be backwards compatible for any existing code.

[kevinburkesegment]

I've fixed everything with CircleCI except the linters; the error message on the linter is incomprehensible.

[petedannemann]

I think we need a separate PR for upgrading the Go version. Since kafka-go is so widely used this will be a breaking change for some users and the more explicit we can make that the less of a surprise it will be to users

[petedannemann]

I think it may be best to test against the same Go version that is in our go.mod file

[petedannemann]

I've fixed everything with CircleCI except the linters; the error message on the linter is incomprehensible.

If you'd prefer I can handle the Go version upgrade and linting fixes. Then this PR will just be the change you actually want to make. Let me know what's best for you

[kevinburkesegment]

If you'd prefer I can handle the Go version upgrade and linting fixes.

Sure, if you have time that would be great.

[jackwilsdon]

I'm not sure this compatibility makes sense - Printf accepts a formatting string and values for the placeholders but slog expects key/value pairs after the first argument and does not do any placeholder substitution.

For example:

kafka-go/reader.go

Line 274 in ebca72e

l.Printf("started commit for group %s\n", r.config.GroupID)

Will cause the following to be logged when using the slog backend:

2009/11/10 23:00:00 INFO started commit for group %s
 !BADKEY=my-group