Enable admin nav feature flag in SAML SSO specs

What does this MR do?

Enables the feature flag group_administration_nav_item in Group SAML SSO specs.

Closes: #213930 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content