Deprecate lib/neutron
Deprecate lib/neutron

lib/neutron-legacy was recently undeprecated (see [0]), Openstack CI
uses neutron-legacy and latest work was done in it also.
To avoid double maintenance lib/neutron can be deprecated.

For latest discussion see [1] and [2].

[0]: https://review.opendev.org/c/openstack/devstack/+/704829
[1]: https://meetings.opendev.org/meetings/networking/2022/networking.2022-01-04-14.04.log.html#l-52
[2]: https://meetings.opendev.org/irclogs/%23openstack-qa/%23openstack-qa.2022-01-05.log.html#t2022-01-05T15:57:37

Related-Bug: #1955765
Change-Id: I3fc328b7f47ccd7c1a97cceeea98fb2fbd609017

openstack-dev/devstack,nawawi/openstack,nawawi/openstack,openstack-dev/devstack