Resolve "Display non-markdown content in the WYSIWYG mode of the SSE::Embedded Ruby"

What does this MR do?

It prevents editing of embedded ruby (ERB) syntax in WYSIWYG mode of the Static Site Editor.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #221079 (closed)