[:monaco_snippets] Enable Monaco as default editor for Snippets

What does this MR do?

Enables Monaco editor for editing Snippets by default. In particular, this MR removes the :monaco_snippets feature flag from the codebase and updates the test specs to be working only with Monaco

Screenshots

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #215975 (closed) and #198608 (closed)