DOC-2363: Custom block elements with colon characters would throw errors.

[kemister85]

Ticket: DOC-2363

Site: Staging branch

Changes:

• add Custom block elements with colon characters would throw errors. to TinyMCE 7.1 release notes.

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/

Review:

• [ ] Documentation Team Lead has reviewed