fix: float <-> decimal coercion should coerce to float

[neilconway]

Which issue does this PR close?

• Closes Coercion between floating point types and exact numeric types should result in floating point type as the common super type #14272

Rationale for this change

Type coercion between a float and a decimal previously resulted in preferring decimal; it should instead prefer float, for several reasons:

1. DataFusion's decimal implementation cannot represent special values like NaN and +/- infinity
2. Float64's range is significantly larger than decimal's range
3. Preferring float matches the behavior of PostgreSQL and DuckDB

What changes are included in this PR?

• Update decimal_coercion behavior
• Update SLT expected results for new behavior
• Add new SLT tests for new behavior

Are these changes tested?

Yes; new tests added.

Are there any user-facing changes?

Yes; changes in type coercion behavior. The migration guide has been updated.