Deprecate `blob` in Snippet GraphQL type

What does this MR do?

Now that we have implemented the new blobs field in the GraphQL Snippet Type, the old blob field can be deprecated.

This MR addresses that by adding the deprecation message and milestone.

Refs #217787 (closed)

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