Deprecate par#

par# primop was supposed to be deprecated in favor of spark# (se #15227 (comment 154293)), however it's still used in GHC.Conc.Sync.par and it's not clear how to replace it with spark# (see !5548 (comment 347791)).

We undeprecated it in !5548 (closed) to support deprecation warnings on primops. If we manage to remove its use in ghc-internal/base, we may consider deprecating it again.

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