[sync] Let new subscribers know about already connected peers (backward-compatible) (#7344)

Revert https://github.com/paritytech/polkadot-sdk/pull/7011 and replace
it with a backward-compatible solution suitable for backporting to a
release branch.

### Review notes
It's easier to review this PR per commit: the first commit is just a
revert, so it's enough to review only the second one, which is almost a
one-liner.

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