hooks: Default-enable synchronized hook executions

In 38e01406 (gitaly/hook: Fix packed-refs lock contention by
synchronizing hooks, 2023-06-13), we have added logic to synchronize the
execution of hooks across the primary and secondary Gitaly nodes. This
is required in order to avoid locking references on the secondaries
while the primary is still busy executing the hook logic.

We have rolled out this change into production systems without any
observed issues so far. Given that this feature flag concerns voting
logic that needs to be the same across all Gitaly nodes we cannot remove
the feature flag immediately though. Instead, we have to release it as
default-enabled first before we can actually remove the flag altogether.
Like this, Praefect continues to decide whether the new voting logic is
being used as it will inject the current flag value into all RPC calls
to Gitaly nodes.