Add automatically updated VS Code config change

The LSP plugin has automatically updated its configuration to a new
format.

.vscode/settings.json

@@ -1,6 +1,8 @@
 {
   // GOV.UK Docker uses `rbenv` to manage Ruby versions
-  "rubyLsp.rubyVersionManager": "rbenv",
+  "rubyLsp.rubyVersionManager": {
+    "identifier": "rbenv"
+  },
   "rubyLsp.formatter": "rubocop",
   "testExplorer.useNativeTesting": true
 }