Add global.json file to lock SDK version (#2104)

Add a global.json file to the project to make it clearer that we should
be using _at least_ version 8.x of the .NET SDK. This can help when
building locally without a valid .NET Runtime version installed, to make
it clear what the issue is.

We set the roll-forward policy to `latestMajor` meaning we will look for
any SDK major version from the set version forward. In this case, since
we set 8.0, this will match 8.x, 9.x, 10.x etc.

When we move to a newer runtime target (probably `net10.0`) we'll want
to bump this minimum SDK version to 10.0.

Author: mjcheetham

global.json

@@ -0,0 +1,7 @@
+{
+  "sdk": {
+    "rollForward": "latestMajor",
+    "version": "8.0"
+  }
+}
+