bundle2: use HG2X in the header
The current implementation of bundle2 is still very experimental and the 3.0
freeze is yesterday. The current bundle2 format has never been field-tested, so
we rename the header to HG2X. This leaves the HG20 header available for real
usage as a stable format in Mercurial 3.1.
We won't guarantee that future mercurial versions will keep supporting this
`HG2X` format.