**What type of PR is this?**
Bug fix
**What does this PR do? Why is it needed?**
Validation of `--backfill-oldest-slot` fails for values > 1056767,
because the validation code is comparing the slot/32 to
`MIN_EPOCHS_FOR_BLOCK_REQUESTS` (33024), instead of comparing it to
`current_epoch - MIN_EPOCHS_FOR_BLOCK_REQUESTS`.
**Which issues(s) does this PR fix?**
Fixes #
**Other notes for review**
**Acknowledgements**
- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).
---------
Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>
