refactor(StateTaxes): flatten form into a single evenly-spaced stack

[aaronlee777]

Summary

• Drop per-requirement-set heading and effective-date subtext from the form
• Render every input as a sibling of one outer Flex with gap={20} so spacing is uniform across the form
• Use flatMap to concatenate requirements across sets while preserving the set-prefixed RHF field key

Test plan

• Open a state taxes form for a state with a single requirement set; verify inputs are stacked with even 20px gaps
• Open a state taxes form for a state with multiple requirement sets; verify inputs from different sets sit as siblings with the same spacing
• Submit the form; confirm the request body still groups requirements by set key

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