Update customer GraphQL doc of deprecation process

What does this MR do?

This MR updates our customer-facing GraphQL documentation to describe the new deprecation/removal process for the GraphQL API.

With this change, the GraphQL API will no longer allow the removal of items on X.6 releases, instead on major releases only. This brings it into line with the wider GitLab breaking changes process:

Identify if deprecating the feature creates a breaking change. If so, you will need to wait until a XX.0 major release for removal.

We retain the existing process of requiring GraphQL schema to be deprecated for at least 6-months before allowing it to be removed (now, only in a major release).

Related issues

#345970 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
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If you are only adding documentation, do not add any of the following labels:

• ~"type::feature"
• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

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