Move Snippets category to Source Code group

Why is this change being made?

Recently, Snippets category was removed from stages.yml in !108078 (merged).

This MR adds Snippets category to Source Code group in stages.yml since Snippets category is now moved to the Source Code Management category from Editor category in !108078 (merged).

Before

After

You can view this change in Review App: https://nh-move-snippets-to-source-code-group-in-stage-yml.about.gitlab-review.app/handbook/product/categories/#categories-a-z

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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