Add canned diagnostics policy for admin users (#8937)

cmd/iam.go

@@ -1392,6 +1392,10 @@ func setDefaultCannedPolicies(policies map[string]iampolicy.Policy) {
 	if !ok {
 		policies["readwrite"] = iampolicy.ReadWrite
 	}
+	_, ok = policies["diagnostics"]
+	if !ok {
+		policies["diagnostics"] = iampolicy.AdminDiagnostics
+	}
 }
 
 // buildUserGroupMemberships - builds the memberships map. IMPORTANT: