Add schema_version to snippets

What does this MR do and why?

This MR sets the schema_version value for the snippets type documents. This field is useful in the case of reindexing all documents of snippets

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

• Ensure Elsaticsearch is enabled.
• Create a new snippet.
• Verify on Elasticsearch that this snippet has a schema_version field set.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #372856 (closed)

#384599 (closed)