[TestFramework] Downgrade Azure.Identity for maximum compat (Azure#18153)

Author: mikeharder

sdk/core/Azure.Core.TestFramework/src/Azure.Core.TestFramework.csproj

@@ -4,7 +4,11 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Azure.Identity" />
+    <!--
+      Test framework should depend on lowest version of Azure.Identity for maximum compatibility, allowing test
+      projects to choose a higher version if desired.
+    -->
+    <PackageReference Include="Azure.Identity" VersionOverride="1.0.0" />
     <PackageReference Include="Newtonsoft.Json" />
     <PackageReference Include="NUnit" />
     <PackageReference Include="NUnit3TestAdapter" />