Merge pull request #4871 from aaronfranke/mono-6.12

Bump required Mono version to 6.12

development/compiling/compiling_with_mono.rst

@@ -8,7 +8,7 @@ Compiling with Mono
 Requirements
 ------------
 
-- Mono 5.12.0 or greater
+- Mono 6.12.0 or greater
 - MSBuild
 - NuGet
 - **On Linux/macOS only:** pkg-config