Draft: First draft at migrating from logrus to slog

This MR has been raised as part of ongoing work to standardise logging across all of our systems at GitLab.

We've determined that slog is going to be the strategic logging library going forward due to its improved performance over Logrus and the fact that it's now baked into the standard library.

This MR migrates all the logrus based log messages emitted from connection.go to slog and removes the logrus dependency entirely from the project.

Note: this cannot be merged as-is due to the fact we lose the ability to output the correlation_id.