Update 2.0 SSO docs with pac4j note

See note for 4.5.6 on pac4j

https://www.pac4j.org/docs/release-notes.html

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Note...

• either stay on SSO 2.x if you stricly use Java 8 at runtime, but be aware that pac4j 4.x is no longer maintained and may not receive security fixes

Future note (add)

• or if you already run magnolia on Java 11+ (majority as per usage metrics), just update to SSO 3.0