v2.1: rolls out chained Merkle shreds to ~5% of mainnet slots (backport of #4019)

[mergify]

Problem

Rolling out chained Merkle shreds to testnet.

Summary of Changes

The commit rolls out chained Merkle shreds to ~5% of mainnet slots.

---

This is an automatic backport of pull request #4019 done by Mergify.

[mergify]

Cherry-pick of d3fe8f8 has failed:

On branch mergify/bp/v2.1/pr-4019
Your branch is up to date with 'origin/v2.1'.

You are currently cherry-picking commit d3fe8f8a0.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   turbine/src/broadcast_stage/standard_broadcast_run.rs

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally