[AlertManager] Handling CMK AccessDenied errors

[alanprot]

What this PR does:
This is a continuation of #5420

Handling access denied errors when cortex lose the access to the CMK (causing access denied on the objstorage).

In those cases AM will crash and fail to start as shown below:

  ts=2023-08-31T19:21:28.327076006Z caller=cortex.go:444 level=error msg="module failed" module=alertmanager err="invalid service state: Failed, expected: Running, failure: failed to load alertmanager configurations for owned users: failed to fetch alertmanager config for user REDACTED: User: REDACTED is not authorized to perform: kms:Decrypt on the resource associated with this ciphertext because the resource does not exist in this Region, no resource-based policies allow access, or a resource-based policy explicitly denies access"

This change is handling the access denied errors the same way AM handler the NotFoundErrors (ignoring)

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]