Update packages used by analyzer tests

Also teach renovate how to find and update these automatically going forward.

Author: AArnott

.github/renovate.json

@@ -52,5 +52,13 @@
 			"matchJsonata": [ "sharedVariableName='CodeAnalysisVersion'" ],
 			"enabled": false
 		}
+	],
+	"customManagers": [
+		{
+			"customType": "regex",
+			"datasourceTemplate": "nuget",
+			"fileMatch": ["test/Microsoft.VisualStudio.Threading.Analyzers.Tests/Helpers/ReferencesHelper.cs"],
+			"matchStrings": ["PackageIdentity\\(\"(?<packageName>[^\"]+)\", \"(?<currentValue>[^\"]+)\"\\)"]
+		}
 	]
 }

test/Microsoft.VisualStudio.Threading.Analyzers.Tests/Helpers/ReferencesHelper.cs

@@ -16,8 +16,8 @@ internal static class ReferencesHelper
 #error Fix TFM conditions
 #endif
         .WithPackages(ImmutableArray.Create(
-            new PackageIdentity("System.Collections.Immutable", "5.0.0"),
-            new PackageIdentity("System.Threading.Tasks.Extensions", "4.5.4"),
+            new PackageIdentity("System.Collections.Immutable", "6.0.0"),
+            new PackageIdentity("System.Threading.Tasks.Extensions", "4.6.0"),
             new PackageIdentity("Microsoft.Bcl.AsyncInterfaces", "6.0.0")));
 
     internal static readonly ImmutableArray<string> VSSDKPackageReferences = ImmutableArray.Create(new string[]