[RBAC] Update "subnet" policies

[Rodolfo Alonso]

* "get_subnet"
Currently only the admin or a project reader can get a subnet. However, it doesn't make sense that the net owner can create the subnet [1] but cannot list it.

* "update_subnet"
Currently only the admin and the network owner can modify the subnet. Any project member should be able too.

* "delete_subnet"
Same argument as in "update_subnet"

[1]https://github.com/openstack/neutron/blob/8cba97016e421e4b01b96de70b4b194972d0186f/neutron/conf/policies/subnet.py#L42-L43

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/897540

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/neutron/+/897540
Committed: https://opendev.org/openstack/neutron/commit/294e1c60b41d3422bb830758e2ea6b6cf554ac46
Submitter: "Zuul (22348)"
Branch: master

commit 294e1c60b41d3422bb830758e2ea6b6cf554ac46
Author: Rodolfo Alonso Hernandez <email address hidden>
Date: Thu Oct 5 19:32:32 2023 +0000

    [RBAC] Update the subnet policies

    * get_subnet: the network owner can retrieve the subnet too.
    * update_subnet: any project member can update the subnet.
    * delete_subnet: any project member can delete the subnet.

    Closes-Bug: #2038646
    Change-Id: Iae2e3a31eb65d68dc0d3d0f9dd9fc8cf83260769

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/neutron 24.0.0.0b1 development milestone.