Declare read_group as the parent permission of GroupStats

What does this MR do and why?

GroupStats skips granular token authorization with parent_authorizes, but names no permission, so it is still listed in authorization_todo.txt. It now declares parent_authorizes: :read_group and leaves the list. This follows !269587 (merged), which authorized GroupReleaseStats.

Why read_group and not read_release:

• Group.stats is its only parent, and Group checks read_group, so no token gains or loses data.
• Its only field, releaseStats, returns GroupReleaseStats, which checks read_release on the group itself since that MR. Declaring read_release on GroupStats would hide stats from a token that reads the group but not its releases. The example 'denying a granular token that only authorizes the parent of a field' in ee/spec/requests/api/graphql/group/release_stats_spec.rb already expects group.stats with read_group alone.

It also pins the directive in the type spec, and updates a comment in group_query_spec.rb that still said releaseStats was not authorized.

References

Related to #639319

Related to #593878

I found this while working on gitlab-mcp-server.

Screenshots or screen recordings

No UI change.

How to set up and validate locally

1. bundle exec rake gitlab:permissions:validate passes. With master's group_stats_type.rb and this todo list, it lists GroupStats as a type that declares no permission.
2. bundle exec rspec ee/spec/graphql/types/group_stats_type_spec.rb ee/spec/requests/api/graphql/group_query_spec.rb ee/spec/requests/api/graphql/group/release_stats_spec.rb passes.

Checks I ran (Ruby 3.3.11)

• The type spec, group_query_spec.rb, release_stats_spec.rb and the GroupReleaseStats type spec: 51 examples, 0 failures, 1 pending.
• With master's group_stats_type.rb, the same specs: 51 examples, 1 failure, the new type spec example.
• gitlab:permissions:validate, RuboCop, the commit linter and Danger's linters pass; both compile_docs tasks leave the committed docs unchanged.

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