global.json: relax rollForward policy to fix CI

Recent fix [1] for our scheduled CI build didn't last long.
Suddenly the "Restore tools" step (`dotnet tool restore`)
started failing in Linux because the GithubActions agent's
dotnet version was apparently upgraded from 9.0.306 to
9.0.307. Given this, the most sensible approach to fix this
is revisit our RollForward policy, which was disabled in a
recent PR [2] because of apparent issues when upgrading our
docs generation.

That issue seems to not be happenning anymore because our
CIs are green even when we upgraded to a .NET version newer
than 9.0.300 (maybe thanks to our Fornax dependency upgrade?),
so it seems safe to restore to a "Minor" setting here.

[1] #780
[2] #736

Author: knocte

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
     "version": "9.0.306",
-    "rollForward": "disable"
+    "rollForward": "minor"
   }
 }