Update FsLexYacc #14583

Hello @vzarytovskii and @KevinRansom, I would like to update the fslex and fsyacc build tools.
The main reason for updating these tools is that the fix of fsprojects/FsLexYacc#154 recently got published to NuGet over at https://github.com/fsprojects/FsLexYacc.

From what I understood the compiler takes a source dependency on FsLexYacc instead of using the NuGet package. What would need to be done to update this?