Enforce owner-only member removal and ownership grants via policy

An organization ADMIN could hard-remove members, including OWNERs, because
removeUser (connect and MCP) only checked the weaker iam:membership-profile:delete
gate. Authorize the owner-only iam:membership:delete instead, and expose the
source attribute on MembershipProfile so the owner grant's non-SCIM condition
can match.

Consolidate ownership-grant authorization into policy for both createUser and
updateMembership: each resolver passes the requested role as a target_role
attribute and ADMIN is denied granting ownership via deny-create-owner /
deny-promote-owner. target_role is distinct from resource.role, which is the
target's current role and guards editing existing owners. With no callers left,
the iam:membership-role:set-owner action (grant and OAuth2 scope) is removed.

Also pass the authorized scope through to the RemoveUser/CreateUser services,
gate the console Remove action on iam:membership:delete, and add regression
tests plus a changelog entry.

Signed-off-by: Sacha Al Himdani <sacha@probo.com>

pkg/iam/iam_actions.go

@@ -48,9 +48,6 @@ const (
 	ActionMembershipUpdate = "iam:membership:update"
 	ActionMembershipDelete = "iam:membership:delete"
 
-	// Membership role actions
-	ActionMembershipRoleSetOwner = "iam:membership-role:set-owner"
-
 	// Membership Profile actions
 	ActionMembershipProfileGet        = "iam:membership-profile:get"
 	ActionMembershipProfileList       = "iam:membership-profile:list"