[CLN] Deprecate blockfile get_at_index()

[Sicheng-Pan]

Description of changes

Summarize the changes made by this PR.

• Improvements & Bug fixes
  • Deprecate blockfile get_at_index method
• New functionality
  • N/A

Test plan

How are these changes tested?

• Tests pass locally with pytest for python, yarn test for js, cargo test for rust

Documentation Changes

Are all docstrings for user-facing APIs updated if required? Do we need to make documentation changes in the docs repository?

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[Sicheng-Pan]

• [CLN] Deprecate blockfile get_at_index() #3332 👈 (View in Graphite)
• [ENH] Implement rank() for blockfile #3331
• main

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[Sicheng-Pan]

Merge activity

• Dec 20, 1:45 PM EST: A user started a stack merge that includes this pull request via Graphite.
• Dec 20, 1:49 PM EST: Graphite rebased this pull request as part of a merge.
• Dec 20, 1:50 PM EST: A user merged this pull request with Graphite.