modem: ppp: gsm: extract access technology

Some modems supports multiple access technologies.
The access technology of the serving cell can be extracted from the
+CEREG command, which is already sent to the modem.

The parse function is extended to also parse the access technology value.

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/51153

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Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>