This changes the engine identifier back to `BLENDER_EEVEE`. We keep the `BLENDER_EEVEE_NEXT` identifier around for versioning reasons (have to detect when it is the active engine of a older file). This also rename a bunch of pannels that were using `next` in their name. This is a breaking change for Addons compatibility. Pull Request: https://projects.blender.org/blender/blender/pulls/140282