Small refactor of ForeignClient::build_update_client* methods

[romac]

Description

• Improve legibility of loop condition when waiting for source chain to reach target_height
• Use Option<Height>::None instead of Height::zero() in ForeignClient::build_update_client*` methods

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For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[romac]

@ancazamfir Do we really want to wait here in a loop? Shouldn't this be handled via retries at the worker level?

[romac]

Let's address this (#1009) all at once in another PR.