Bump required Mono version to 6.12

(cherry picked from commit 673aa368cc03e3b85b4cd89b1f25f379e23909f6)

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