Fix note creation error for SHA256 repositories

What does this MR do and why?

Contributes to #463793 (closed)

Problem

Note model uses a json validation to verify the schema of the position field. This validation fails because SHA256 repository generates commits with a length 64 instead of standard 40.

Solution

Extend validation to accept SHA256 commit hashes with length 64.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• [Sha 256] Unable to add note to MR diff when us... (#463793 - closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Enable feature flag Feature.enable(:support_sha256_repositories) in Rails console
2. Create a new project with SHA256 repository

3. Make changes (update file for example) and create a new branch with a merge request
4. Try to leave a comment on the merge request changes (select a line of code, not just a regular comment)
5. It should work without any errors