[SecuritySolution][Timelines] Update privilege (elastic#208383)

## Summary

We forgot to update this privilege in
elastic#201780 . The endpoint only uses
the scoped SO client, so this missing privilege declaration does not
lead to privilege escalation on the endpoint. There are automated tests
that check for the correct privilege access for this and other
endpoints.

x-pack/solutions/security/plugins/security_solution/server/lib/timeline/routes/timelines/delete_timelines/index.ts

@@ -22,7 +22,7 @@ export const deleteTimelinesRoute = (router: SecuritySolutionPluginRouter) => {
       path: TIMELINE_URL,
       security: {
         authz: {
-          requiredPrivileges: ['securitySolution'],
+          requiredPrivileges: ['timeline_write'],
         },
       },
       access: 'public',