Release post items: update PMs checklist to facilitate checking each item individually

Why is this change being made?

Although keeping (almost) the same items, separating each checklist item individually to facilitate checking the items and avoid them to slip through.

Added a couple of items:

• Content:

  • ...

  • Ensure stage has the correct format.

  • Ensure categories have the correct format.

Preview: https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/rp-update-pms-checklist/.gitlab/merge_request_templates/Release-Post-Item.md#pm-release-post-item-checklist

Follows !42566 (merged)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what