Docs: add callout about legacy allow_client_traits evaluation behaviour

[khvn26]

Part of #6822

Changes

In docs/administration-and-security/governance-and-compliance/security.md, add an admonition explaining:

• Environments created before the fix with this setting disabled may still use client-sent traits for segment evaluation (legacy behaviour).
• Re-saving the setting in the dashboard opts into the correct, more secure behaviour.

Also update PR #6782.

Key files

• docs/docs/administration-and-security/governance-and-compliance/security.md
• docs/docs/flagsmith-concepts/segments/index.md

Depends on

• Backend: Add granular trait control for client-side SDKs #6823
• https://github.com/Flagsmith/edge-api/issues/550

[khvn26]

Merged into #6824.