[Email Security] Add post-quantum key exchange changelog

[akumar0630]

Adds a changelog entry announcing post-quantum hybrid key exchange support (X25519MLKEM768) on Email Security's inbound MX and outbound delivery SMTP connections.

Highlights

• Inbound MX and outbound delivery connections negotiate the X25519MLKEM768 hybrid key agreement when the peer supports it, protecting SMTP traffic against harvest-now, decrypt-later attacks.
• Enabled automatically for all customers; backwards compatible with peers that do not yet advertise post-quantum key agreement.
• Applies to Advantage, Enterprise, and Enterprise + PhishGuard packages.

Validation

• pnpm run format:core:check passes
• pnpm run check not run locally due to Node version mismatch (CI will validate)

[cloudflare-docs-bot]

Review

✅ No issues found in commit 8f1b621.

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[goldbe-cf]

looks good thanks