builtins: update builtin versions in examples

[simevo]

see eclipse-theia/vscode-builtin-extensions#61

example for theia-electron/package.json:

-    "vscode-builtin-css": "https://github.com/theia-ide/vscode-builtin-extensions/releases/download/v1.39.1-prel/css-1.39.1-prel.vsix",
+    "vscode-builtin-css": "https://open-vsx.org/api/vscode/css/1.56.2/file/vscode.css-1.56.2.vsix",

[vince-fugnitto]

The examples in the repository are in fact examples, we will probably want to update the list of builtins once extensions-packs as builtins are supported (which will make the maintenance much easier).

A similar idea was proposed for theia-blueprint: eclipse-theia/theia-blueprint#93.

[stale]

This contribution has been automatically marked as stale due to inactivity, and it will be closed if no further activity occurs. Thank you for contributing to Theia!