Have the flag enable-protocol-upgrade-via-soroban-env-host-prev set by default

[anupsdf]

What:

Have the flag enable-protocol-upgrade-via-soroban-env-host-prev set by default. Introduce a new flag to exclude prev so it can be used for minor releases after the protocol upgrade.

Why:

Nodes without the prev host will reject protocol upgrades during consensus. During protocol 21 upgrade we saw a validator had built core from source and missed setting this flag. This caused them to loose sync during the upgrade.

[sisuresh]

I wonder if we should just remove the flag and always have prev enabled. @graydon Is there any benefit to removing the prev crate other than slightly reducing the binary size?

[graydon]

Nothing comes to mind, I'd certainly be willing to try it and see if anything goes wrong.

[sisuresh]

Closed by #4434