Remove snippet content search

What does this MR do?

Remove snippet content search. Put API deprecation notice.

After snippet versioning is enabled, we will look to see how to enable it in Elasticsearch.

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.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #199220 (closed)