Added fallback_max_weight to Transact for sending messages to V4 chai… (#6778)

Backport of https://github.com/paritytech/polkadot-sdk/pull/6643 to
stable2412.

We need it before the release so we have a good story with chains that
haven't upgraded yet.

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Co-authored-by: GitHub Action <[email protected]>
Co-authored-by: Adrian Catangiu <[email protected]>
Co-authored-by: Branislav Kontur <[email protected]>