fix: permission checks for editing access keys (#18928)

With this change, only a user with `UpdateServiceAccountAdminAction`
permission is able to edit access keys.

We would like to let a user edit their own access keys, however the
feature needs to be re-designed for better security and integration with
external systems like AD/LDAP and OpenID.

This change prevents privilege escalation via service accounts.

cmd/auth-handler.go

@@ -162,7 +162,8 @@ func validateAdminSignature(ctx context.Context, r *http.Request, region string)
 	s3Err := ErrAccessDenied
 	if _, ok := r.Header[xhttp.AmzContentSha256]; ok &&
 		getRequestAuthType(r) == authTypeSigned {
-		// We only support admin credentials to access admin APIs.
+
+		// Get credential information from the request.
 		cred, owner, s3Err = getReqAccessKeyV4(r, region, serviceS3)
 		if s3Err != ErrNone {
 			return cred, owner, s3Err