EEVEE: Remove all remaining reference of EEVEE next

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

tests/python/workbench_render_tests.py

@@ -94,7 +94,7 @@ def main():
     if args.gpu_backend == "vulkan":
         report.set_compare_engine('workbench', 'opengl')
     else:
-        report.set_compare_engine('eevee_next', 'opengl')
+        report.set_compare_engine('eevee', 'opengl')
     report.set_pixelated(True)
     report.set_reference_dir("workbench_renders")