Backport of scheduler: fix reconciliation of reconnecting allocs into release/1.5.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #16609 to be assessed for backporting due to the inclusion of the label backport/1.5.x.

The below text is copied from the body of the original PR.

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When a disconnect client reconnects the allocReconciler must find the
allocations that were created to replace the original disconnected
allocations.

This process was being done in only a subset of non-terminal untainted
allocations, meaning that, if the replacement allocations were not in
this state the reconciler didn't stop them, leaving the job in an
inconsistent state.

This inconsistency is only solved in a future job evaluation, but at
that point the allocation is considered reconnected and so the specific
reconnection logic was not applied, leading to unexpected outcomes.

This commit fixes the problem by running reconnecting allocation
reconciliation logic earlier into the process, leaving the rest of the
reconciler oblivious of reconnecting allocations.

It also uses the full set of allocations to search for replacements,
stopping them even if they are not in the untainted set.

The system SystemScheduler is not affected by this bug because
disconnected clients don't trigger replacements: every eligible client
is already running an allocation.

Closes #15483