Not able to install example submodule

@glottus , we are getting this error because the migrate_google_sheets_example_setup module is trying to install a block configuration that depends on the Bartik theme, which is no longer available in Drupal 11.
There are two ways we can fix this :

1. Change the bartik theme to any other theme
2. Moving the block configuration from config/install to config/optional

I am doing the 2 option so that we don't get this error.