DOC-2456: Docs 6 - Update DOMPurify version in security guide

[FarzadHayat]

Ticket: DOC-2456

Site: Staging branch

The version of DOMPurify used in TinyMCE was updated to 3.x in this commit which was released with TinyMCE 6.8.0, however, the documentation was never updated to match this.

Changes:

• Update DOMPurify version in security guide to explain version change from DOMPurify 2.x to 3.x in TinyMCE 6.8.0
• Replace some TinyMCE 6 references with Antora attributes

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/
• [-] modules/ROOT/nav.adoc has been updated (if applicable)
• [-] Files has been included where required (if applicable)
• [-] Files removed have been deleted, not just excluded from the build (if applicable)
• [-] Files added for New product features, and included a release note entry.

Review:

• Documentation Team Lead has reviewed