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 extension), 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. -- https://gerrit.wikimedia.org/r/1184914
• 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. -- https://gerrit.wikimedia.org/r/1184915
• 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, we're ready to enable external traffic routing to -next. This will involve setting enabled = true and load_fraction = 1.0 in mw-next-routing.lua.conf. Doing so should wait until shortly before we begin the external traffic ramp.