Add schema_version to milestones

What does this MR do and why?

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

Screenshots or screen recordings

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Before	After

How to set up and validate locally

• Ensure Elsaticsearch is enabled.
• Create a new milestone or update an existing milestone.
• Verify on Elasticsearch that this milestone has a schema_version gets 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)

#384595 (closed)