Convert lib/utils to slog

[rosstimothy]

This only converts the places in the utils package that were logging with logrus, but it does not fully remove the logrus package as a dependency. The package is home to the logrus text formatter which needs to exist until slog is the only logging mechanism in use.

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[kopiczko]

Mostly LGTM, but those context.Backgroud() calls outside test files are a bit concerning. It may be less obvious to replace them with a proper context even if the calling function eventually takes it as an argument. But possibly that's something already discussed?

[kopiczko]

"Backends added"?

[rosstimothy]

Per RFD 154:

Use the Context variants of the slog.Logger API. Providing the context.Context to all log messages allows them to be enriched with additional attributes. For example, this allows log messages to automatically have a request or trace id added, which makes correlating messages that were emitted as a result of one particular action or event. If no context is readily available for the logger to consume, prefer to use a context.TODO if it is possible to get a context to the call site in the future but refactoring is to do be done in a separate unit of work. If there is likely to never be a context provided to the function producing the log output, then prefer to use a context.Background to indicate as such