[msbuild] Demote W7091 to a message instead of warning. (#22000)

This warning seems to confuse people more than help them, so stop emitting it
as a warning. Keep it as a message though, so it shows up in binlogs:

> path/to/myframework.framework is a framework of static libraries, and will not be copied to the app.

---------

Co-authored-by: Alex Soto <[email protected]>

Author: rolfbjarne

msbuild/Xamarin.MacDev.Tasks/Tasks/FilterStaticFrameworks.cs

@@ -52,7 +52,7 @@ public override bool Execute ()
 						continue;
 					}
 
-					Log.LogWarning (7091, frameworkExecutablePath, MSBStrings.W7091 /* "The framework {0} is a framework of static libraries, and will not be copied to the app." */, Path.GetDirectoryName (frameworkExecutablePath));
+					Log.LogMessage (MessageImportance.Low, MSBStrings.W7091 /* "The framework {0} is a framework of static libraries, and will not be copied to the app." */, Path.GetDirectoryName (frameworkExecutablePath));
 					list.RemoveAt (i);
 				}