Merge pull request dotnet#59248 from CyrusNajmabadi/langUpgradePreview

Always allow the 'upgrade project' fixer to upgrade to 'preview' versions of C#

src/VisualStudio/CSharp/Impl/Utilities/CSharpParseOptionsChangingService.cs

@@ -38,6 +38,15 @@ public bool CanApplyChange(ParseOptions oldOptions, ParseOptions newOptions, str
             {
                 return true;
             }
+            else if (newCSharpOptions.LanguageVersion == LanguageVersion.Preview)
+            {
+                // It's always fine to upgrade a project to 'preview'.  This allows users to try out new features to see
+                // how well they work, while also explicitly putting them into a *known* unsupported state (that's what
+                // preview is after all).  Importantly, this doesn't put them into an unrealized unsupported state (for
+                // example, picking some combo of a real lang version that isn't supported with their chosen framework
+                // version).
+                return true;
+            }
             else
             {
                 Contract.ThrowIfFalse(LanguageVersionFacts.TryParse(maxLangVersion, out var parsedMaxLanguageVersion));