Update docs milestone description for alpha

`alpha` deprecations were added in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84756 as a hackish
way to denote an alpha field in GraphQL.

This change updates the generated docs to no longer say:
"Deprecated in <milestone>" for these kinds of deprecations.

Partly https://gitlab.com/gitlab-org/gitlab/-/issues/336383

Changelog: changed