Specify tiers for SAML SSO at self-hosted plans

Users can use SAML SSO via OmniAuth only in self-hosted
from Core and higher tiers, not on .com SaaS.
Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

changelogs/unreleased/docs-saml-sso-from-core-at-self-hosted.yml

+---
+title: Specify tiers for SAML SSO at self-hosted plans
+merge_request: 34040
+author: Takuya Noguchi
+type: other

doc/integration/saml.md

-# SAML OmniAuth Provider
+# SAML OmniAuth Provider **(CORE ONLY)**
 
 Note that: