Use language version 2.0 for extensibility (#17714)

## Description

Use language version 2.0 for extensibility

## Checklist

- [x] I have read and adhere to the [contribution
guide](https://github.com/Azure/bicep/blob/main/CONTRIBUTING.md).

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/Azure/bicep/pull/17714)

Author: anthony-c-martin

src/Bicep.Core.IntegrationTests/ExtensionRegistryTests.cs

@@ -388,13 +388,13 @@ public async Task Third_party_imports_are_enabled_when_feature_is_enabled()
         result.Template.Should().DeepEqual(JToken.Parse("""
 {
   "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
-  "languageVersion": "2.1-experimental",
+  "languageVersion": "2.0",
   "contentVersion": "1.0.0.0",
   "metadata": {
     "_generator": {
       "name": "bicep",
       "version": "dev",
-      "templateHash": "15182588323302093073"
+      "templateHash": "9550648641475979632"
     }
   },
   "imports": {