GraphQL: Add alpha deprecation reason

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR renames the :discouraged reason into :alpha (meaning that an API is in Alpha state and should not be relied on). The :discouraged reason was not being used anywhere, so :alpha can take its place. This is meant to be used in situations where the use of a Feature Flag is not desired.

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https://gitlab.slack.com/archives/C6MLS3XEU/p1647624368906019