Discontinue Domain Manager standard once OpenStack 2024.2 lands in SCS

[markus-hentsch]

Current State

• we merged the Draft of the SCS Domain Manager standard back in https://github.com/SovereignCloudStack/standards/pull/343
• we almost concluded upstreaming the Domain Manager via https://github.com/SovereignCloudStack/issues/issues/384 and it will be a native feature in OpenStack 2024.2
• the SCS R7 release will still be based on 2024.1 and lack native integration of this feature, so it still needs the standard for the time being
  • there is now a https://github.com/SovereignCloudStack/standards/pull/586 PR to stabilize the standard so that the feature can become available regardless of the OpenStack version

Desired State

Once SCS incorporates OpenStack 2024.2 or later, where the Domain Manager functionality is available natively in OpenStack, the following things need to happen:

1. The SCS Domain Manager standard needs to be deprecated. It needs to either be removed or replaced by a v2 that omits any of the SCS-specific policy configuration, as it will not be necessary anymore and conflict with the native implementation. Explanations about is_domain_managed_role and an Octavia role integration could be remaining parts of a v2 although I think it would better fit in the Role Standard. I see little reason for keeping the standard at all. Update: #586 now differentiates between pre- and post-2024.2 versions of Keystone. As such, the standard can live on for the time being. In the future, all the pre-2024.2 parts can be removed.
2. The docs page at docs/05-iam/domain-manager-setup-and-usage.md needs to be updated and reference to the Domain Manager standard be removed or replaced in a way that omits the SCS-specific policy configurations. Only the part about adjusting the is_domain_managed_role rule should stay as this will still be valid and required for the Octavia LBaaS role integration.
3. SCS operators need to be notified to remove the Keystone policy extensions of the Domain Manager standard when upgrading to the SCS release that incorporates OpenStack 2024.2 or later.

[markus-hentsch]

2. The docs page at docs/05-iam/domain-manager-setup-and-usage.md needs to be updated and reference to the Domain Manager standard be removed or replaced in a way that omits the SCS-specific policy configurations. Only the part about adjusting the is_domain_managed_role rule should stay as this will still be valid and required for the Octavia LBaaS role integration.

We might be able to greatly shrink this down and simply point to https://docs.openstack.org/keystone/latest/user/index.html depending on how https://review.opendev.org/c/openstack/keystone/+/928135 goes.