NEWS: Add entry for the deprecation of bst checkout

buildstream 1.3.1

=================

   o BREAKING CHANGE: Moved `checkout` to the artifact subcommands. `bst checkout` is

     now obsolete.

     The command is now `bst artifact checkout`. `bst checkout` has been deprecated.

     This implements the features of `bst checkout`, although the mandatory LOCATION

     argument should now be specified with the `--directory` option.

     `artifact checkout` can handle both element names and artifact refs, of which it

     can take multiple.

     `--deps`, `--tar` and `--integrate` options are only supported when passed a

     single ELEMENT target.

  o Added `bst artifact log` subcommand for viewing build logs.

  o BREAKING CHANGE: The bst source-bundle command has been removed. The