Simplify state transactions function in FSM

[Stefan-Ethernal]

Description

As stateTransactions function returns only single transaction, which is the one for bridge commitment registration, it is renamed and it's signature is changed, so it returns only single instance instead of slice.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Please complete this section if you ran manual tests for this functionality, otherwise delete it

Documentation update

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Additional comments

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[goran-ethernal]

Except for a small comment, LGTM 💯