Remove stage definition in API Fuzzing

Problem

The API fuzzing template defines stages for customers. This is overly prescriptive, and will overwrite their stages if they include the template after the stages are already defined in their .gitlab-ci.yml. This is also inconsistent with coverage guided fuzzing.

Proposal

1. Remove

stages:
    - build
    - test
    - deploy
    - fuzz

from the API-Fuzzing.gitlab-ci.yml template. !59154 (merged) remove it from API-Fuzzing.latest.gitlab-ci.yml

2. Update the documentation that states customers must define a fuzz stage in their stages. !58792 (merged)

3. Update the API Fuzzing merge request snippet tool to generate the text

stages:
    - fuzz

4. This is a breaking change, and should be announced to customers. !78796