chore: interface{} -> any in comments

[SoloJacobs]

Tiny left over from https://github.com/prometheus/alertmanager/pull/4750

(I noticed after rebasing on @SuperQ changes)

[SoloJacobs]

Hmm, I am a bit confused about this:

$ head api/v2/restapi/configure_alertmanager.go
// This file is safe to edit. Once it exists it will not be overwritten

I feel about not chaning api/v2 anyway, so I dropped the change.