Title case should be used for categories

Why is this change being made?

@marcia pointed out to me that case is important (code review vs Code Review) in release post items !38272 (comment 271803002).

Example output from running: bin/release-post-item --dry-run https://gitlab.com/gitlab-org/gitlab/issues/29782

---
feature:
  secondary:
  - name: S/MIME X509 verification of commits
    available_in:
    - core
    - starter
    - premium
    - ultimate
    gitlab_com: true
    documentation_link: https://docs.gitlab.com/ee/#amazing
    image_url: "/images/unreleased/feature-a.png"
    reporter: jramsay
    stage: create
    categories:
    - Source Code Management
    issue_url: https://gitlab.com/gitlab-org/gitlab/issues/29782
    description: |
      Lorem ipsum [dolor sit amet](#link), consectetur adipisicing elit.
      Perferendis nisi vitae quod ipsum saepe cumque quia `veritatis`.

Does this MR meet the acceptance criteria?

Conformity

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