When using `schedule_code_upgrade` to change the code of a parachain in
the relay chain runtime, we had already fixed to not set the `GoAhead`
signal. This was done to not brick any parachain after the upgrade,
because they were seeing the signal without having any upgrade prepared.
The remaining problem is that the parachain code is only upgraded after
a parachain header was enacted, aka the parachain made some progress.
However, this is quite complicated if the parachain is bricked (which is
the most common scenario why to manually schedule a code upgrade). Thus,
this pull request replaces `SetGoAhead` with `EnactUpgradeDirectly` to
signal to the logic that we don't want to set the go ahead signal and
that we want to replace the current parachain code directly without
waiting for some progress of the parachain.

…dk into bkchr-go-ahead-fix