(Tentatively) Fix "SAML for groups" tiers

• False in self-managed
• True only in Silver and Gold
• Fix logic in lib/feature.rb: features present in gitlab.com aren't missing.

Review apps:

• Features: https://update-saml-groups-features-yml.about.gitlab-review.app/features/
• Comparison: https://update-saml-groups-features-yml.about.gitlab-review.app/pricing/self-managed/feature-comparison/