Update RP post required fields and label requirements
# Problem to solve Some release posts were missing required fields. These missing fields were not being caught by the MR pipeline and had to be resolve after content assembly. Additionally, in the 18.4 cycle a new `Dedicated` badge along with `gitlab_dedicated` field was added. This field was updated manually by the RPM in this cycle but it should move to the individual PMs to determine if their feature is available in Dedicated and correctly fill in the field on future RPs. # Proposal 1. Confirm and add if necessary the following fields as required: * `gitlab_com` * `self_managed` * `gitlab_dedicated` * `available_in` with additional requirement that this cannot be contain an empty array. * `documentation_link` * `issue_url` or `epic_url` 2. Require that the RP MR has one of the following labels in accordance with the type of MR it is: * ~"release post item::primary" * ~"release post item::top" * ~"release post item::secondary" * ~"release post item::upgrades" * ~"release post item::extras" * ~"release post item::removal" This makes it easier for the RPM to filter and review different categories of RPs. 3. An attempt should also be made to review and perform any updates to [handbook page section for available_in guidance](https://handbook.gitlab.com/handbook/marketing/blog/release-posts/#feature-availability). ## Implementation 1. [x] Updated required fields in linter: https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/141182 2. [x] Add policy to https://gitlab.com/gitlab-org/quality/triage-ops to ping assignee when missing `release post item::` type label and `release post item` is present: https://gitlab.com/gitlab-org/quality/triage-ops/-/merge_requests/3684 3. [x] Update handbook as necessary: https://gitlab.com/gitlab-com/content-sites/handbook/-/merge_requests/16460
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