Relax 'any' index signature requirements when a type contains a string index signature to 'any'

[DanielRosenwasser]

Daniel Rosenwasser (@DanielRosenwasser) Wes and I spoke and recommend that we keep the assignability-target rule in place when the target has both string and numeric index signatures

Originally posted by Ryan Cavanaugh (@RyanCavanaugh) in #43004 (comment)