TINYDOC-3631: TinyMCE 7.9.4 security release notes.

[kemister85]

Ticket: TINYDOC-3631

Site: Staging branch

Changes:

• Add 7.9.4-release-notes.adoc with the media plugin data-mce-object serialization XSS fix (GHSA-mf2p-h6hf-fcwm); CVE pending.

Pre-checks:

• Branch prefixed with feature/7/ or hotfix/7/
• modules/ROOT/nav.adoc has been updated (if applicable)
• Files has been included where required (if applicable)
• Files added for New product features, and included a release note entry.

Review:

• Documentation Team Lead has reviewed