gnss: rename u-blox M10 driver to M8

The driver in tree is for u-blox M8 devices, not M10. The M10 series
devices (from Protocol Version 23.01) use a different, non backwards
compatible interface for configuring the modem behaviour.

Of the two boards tested in the original PR, the "VMU RT1170" is
explicitly listed as having a u-blox NEO-M8N modem, while I have
been unable to find any information online about the "FMURT6" board.

Leaving the naming as-is will cause problems when M10 drivers are
contributed.

Signed-off-by: Jordan Yates <jordan@embeint.com>