Update korebuild and VSIX generation settings to support 15.8 VS.

- Prior to this if a 15.8 VS was installed the VSIX creation target would fail due to a bug in the CLI we're currently pinned to (already fixed, we just haven't pulled it). The fix was removing the parallelism flags to msbuild to allow it to run unrestricted in regards to processor utilization.
- Changed the VS restrictions to ensure we don't use unsupported 15.9 bits that have breaking changes in their MSBuild bits (this has proven to be troublesome in the past).

build/VSIX.targets

@@ -63,7 +63,6 @@
       <MSBuildArguments Include="
         $(VSIXProject);
         /t:Restore;
-        /m;
         /v:m;
         /p:Configuration=$(Configuration);
         /p:BuildNumber=$(BuildNumber);" />
@@ -96,7 +95,6 @@
       <MSBuildArguments Remove="@(MSBuildArguments)" />
       <MSBuildArguments Include="
         $(VSIXProject);
-        /m;
         /v:M;
         /fl;
         /flp:LogFile=$(VSIXLogFilePath);

korebuild.json

@@ -5,7 +5,7 @@
     "visualstudio": {
       "required": false,
       "includePrerelease": true,
-      "minVersion": "15.0.26730.03",
+      "versionRange": "[15.0.26730.03, 15.9)",
       "requiredWorkloads": [
         "Microsoft.VisualStudio.Component.VSSDK"
       ]