Remove support for `deprecated: { reason: :alpha }`

What does this MR do and why?

This MR removes support in our GraphQL deprecations for creating an alpha/experimental schema item through:

deprecated: { reason: :alpha, ... }

We now only support this method:

alpha: { ... }

The former was the original way that alpha schema was introduced. Subsequently, in %15.3 we introduced the alpha: keyword !90766 (merged) and documentation.

This MR also changes the remaining old uses of the deprecated: { reason: :alpha } style to use alpha:.

How to set up and validate locally

There is no change to the generated GraphQL docs which include labels for alpha/experimental items:

On this branch, the following rake task should make no changes:

bundle exec rake gitlab:graphql:compile_docs

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.