Migrate mason-lspconfig handlers to vim.lsp.config

[ReadHegel]

As stated in the mason-lspconfig changelog, they removed the handlers configuration in their plugin, making Kickstart no longer responsible for automatic LSP server configuration.

### Removed Features

- Removed the `handlers` setting and `.setup_handlers()` function.
- Replaced by the new native `vim.lsp.config()` API.
- Introduced a new `automatic_enable` setting.