Entity Framework Core DbFunction for square root
Here I capture the main essence of the comments.
Facts

As the OP has mentioned the Sqrt functionality is no exposed on the EF.Functions in EFCore

EF 6 exposed it through SqlFunctions


Math.XYZ functions can be used in Linq2Entities

Not all of the functions have translator to the corresponding SQL function, current state
As a part of this issue more function translators will in v5



Solutions

Math.Pow can be used as well, like this: Math.Pow(a.someColumnWithDoubleValue, 0.5)

This will be translated into POWER


Math.Sqrt can be used as well, like this: Math.Pow(a.someColumnWithDoubleValue)

This will be translated into SQRT
If the data type would be decimal then explicit cast is needed:

Math.Pow((double)a.someColumnWithDecimalValue)
Translated into SQRT(CONVERT(Float, ...)