Add milestone/workflow policy for TI and CS

What does this MR do and why?

Add a new policy for groupthreat insights and ~"group::container security" that unassigns everyone from issues that are workflowready for development but don't have a release milestone (i.e. nn.n).

Action from the 14.8 retro for Threat Insights: https://gitlab.com/gl-retrospectives/secure-sub-dept/threat-insights/-/issues/10#note_851853824

Expected impact & dry-runs

Local dry-run using policies/groups/gitlab-org/threat-insights/remove-assignee.yml

• Found 65 resources...
• Filtering resources...
• Total after filtering: 2 resources
• Limiting resources...
• Total after limiting: 2 resources

The following comments would be posted for the rule Unassign issues without a release milestone that are ready for development:

gitlab-org/gitlab#336091

@ismailarilik

You have been unassigned from this issue because it's marked as ~"workflow::ready for development" but the milestone is not set to a release.

[Bot policy](https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/policies/groups/gitlab-org/threat-insights/remove-assignee.yml).

/unassign @ismailarilik

gitlab-org/gitlab#299904 (closed)

@thiagocsf @minac

You have been unassigned from this issue because it's marked as ~"workflow::ready for development" but the milestone is not set to a release.

[Bot policy](https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/policies/groups/gitlab-org/threat-insights/remove-assignee.yml).

/unassign @thiagocsf @minac

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review