chore: deprecate X=Mimir-SkipLabelNameValidation once UTF-8 is supported

[ywwg]

https://github.com/grafana/mimir/blob/main/pkg/api/api.go#L72

This flag will be redundant to the UTF-8 feature and can be removed once UTF-8 is supported in mimir. We can mark the flag as deprecated and perhaps remove the code. Or, just have the old flag trigger the new behavior