Disable rollForward for SDK in global.json (#170)

Update .NET SDK to the latest released version and disable
`rollForward`.

Updates to the .NET SDK feature band can introduce new / updated
analyzers (for instance, 8.0.400 adds
[IDE0320](dotnet/roslyn#73012)). As a result,
updating the feature band isn't "safe" to do from a build
reproducibility perspective. It will also conflict with #166.

That means the roll forward options available to us are:

- `patch`
- `latestPatch`
- `disable`

However, GitHub Actions don't support these options (tracked by
actions/setup-dotnet#448). Thus, the only
`rollForward` strategy that currently does the same thing locally and in
CI is `disabled`.

Once 448 is fixed, we can / should probably switch to `latestPatch`
(tracked by #171).

This 'gotcha' is also added to the SquiggleCop documentation
[here](https://github.com/MattKotsenas/SquiggleCop?tab=readme-ov-file#common-sources-of-baseline-mismatches).

global.json

@@ -1,7 +1,7 @@
 {
   "sdk": {
-    "version": "8.0.101",
-    "rollForward": "latestFeature"
+    "version": "8.0.303",
+    "rollForward": "disable"
   },
   "msbuild-sdks": {
     "DotNet.ReproducibleBuilds.Isolated": "1.2.4"