Core bump 2021

[vbfox]

Bump FSharp.Core to it's 2021 edition with support for interpolated strings

TODO:

• Add more tests of each case
• Document how Interpolated string holes and format specifiers ends up as PrintableElement
• Finds out what should be our version dependencies
• Compile via expression trees instead of doing reflection when the env allow compilation

[jwosty]

Is there anything I can do to help get this through? I would really love to be able to interop with interpolated strings and the new format specifiers (%B, %o, etc) in custom printf functions.