Add fallback_max_weight to snowbridge Transact (#6792)

We removed the `require_weight_at_most` field and later changed it to
`fallback_max_weight`.
This was to have a fallback when sending a message to v4 chains, which
happens in the small time window when chains are upgrading.
We originally put no fallback for a message in snowbridge's inbound
queue but we should have one.
This PR adds it.

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Co-authored-by: GitHub Action <action@github.com>