RateLimitingMiddleware still belongs to v2 API code

[Takashi Kajinami]

# This might not be the best place to discuss this but I hope this can initiate some discussion.

Currently RateLimitingMiddleware belongs to v2 API code, which is located under cinder.api.v2.
Ideally we should migrate this to v3 API code but there are some points we should consider before we determine to keep that middleware.

- The middleware is not included in the pre-defined pipelines in api-paste.ini .

- The middleware was ported from Nova but Nova has already dropped the middleware

- The middleware uses local memory to count API frequency. Which means it doesn't work properly in deployments with multiple nodes hosting cinder-api.

Considering these situations, it'd make more sense to deprecate/remove the middleware instead of keeping it under v3 API code.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/cinder/+/807469

[Sofia Enriquez (lsofia-enriquez)]

This bug is related to https://bugs.launchpad.net/cinder/+bug/1662637

[Brian Rosmaita (brian-rosmaita)]

As a comment on https://review.opendev.org/c/openstack/cinder/+/807469 indicates, some operators are still using this. So it looks like we do need to migrate it to the cinder.api.v3 module.

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Takashi Kajinami <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/cinder/+/807469
Reason: The middleware is still used, thus can't be removed. We should migrate the code to v3 code base.