GraphQL: remove deprecated method of decoding cursors

The format of GraphQL cursors changed from a straight encoded string into an encoded JSON string (!17788 (merged))

This change could cause clients to initially fail if they request the old style cursor, so we left in some backward compatibility. However once a client request succeeds, it will get the new cursor format.

We simply need to remove this support in next release (12.5)

A TODO has been placed in these files:

• https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/graphql/connections/keyset/connection.rb
• https://gitlab.com/gitlab-org/gitlab/blob/master/spec/lib/gitlab/graphql/connections/keyset/connection_spec.rb