In advance of the PHP 8.3 migration, we'll need to ensure that the presence of the `PHP_ENGINE=8.3` Cookie value (to be set by the WikimediaEvents plugin), results in requests being routed to the `-next` releases of mw-web and mw-api-ext.
Following the changes in T391421, the steps are slightly different than when we first introduced `-next` routing for the 8.1 migration (T377042):
[ ] Ensure `mw-next-routing` is wired into `profile::trafficserver::backend` as a `trafficserver::lua_script` resource.
[ ] Ensure `mw-next-routing.lua` is wired into the appropriate position (immediately after `multi-dc.lua`) in the plugin chains for the `profile::trafficserver::backend::mapping_rules` entries targeting mw-web and mw-api-ext.
[x] Ensure `mw-next-routing.lua.conf` targets `"PHP_ENGINE=8.3"` via `cookie_pattern` -- This was done as part of the restructuring in https://gerrit.wikimedia.org/r/1154901.
Once all of these steps are complete and we're ready to enable external traffic routing to `-next`, we can set `enabled = true` in `mw-next-routing.lua.conf`.