TINY-10676: Update reference of TinyMCE 6 to 7

[ArvinJ-H]

Related Ticket: TINY-10676

Description of Changes: Update reference of TinyMCE 6 to 7 in root, oxide and oxide-icons-default

Pre-checks:

• [x] Changelog entry added
• [x] Tests have been added (if applicable)
• [x] Branch prefixed with feature/, hotfix/ or spike/

Review:

• [x] Milestone set
• [x] Docs ticket created (if applicable)

GitHub issues (if applicable):