Init SLogger in initLogger function

[SungJin1212]

This PR updates the InitLogger function to initialize the global SLogger variable using GoKitLogToSlog(Logger). previously, SLogger was initialized as a NopLogger and not updated, making it unusable for actual logging.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

previously, SLogger was initialized as a NopLogger and not updated, making it unusable for actual logging.

Where did we initialize Slogger as a noop before?

[SungJin1212]

@yeya24
The Slogger was updated globally in https://github.com/cortexproject/cortex/blob/master/pkg/util/log/log.go#L20.