This PR updates the language server configuration for Elixir and HEEx to
not start the [Expert](https://github.com/elixir-lang/expert) language
server by default.

While Expert is the official Elixir language server, it is still early,
so we don't want to make it the default just yet.

Release Notes:

- Updated the default Elixir and HEEx language server settings to not
start the Expert language server.