cephadm: fix regexp to strip `v1:` or `v2:` prefix from IPv6 addr
regexp was striping the first hextet of the IPv6 address:
```
FAILED tests/test_cephadm.py::TestBootstrap::test_mon_addrv[[0000:0000:0000:0000:0000:FFFF:C0A8:0101:1234]-list_networks5-None] - cephadm.Error: Cannot infer CIDR network for mon IP `0000:0000:0000:0000:FFFF:C0A8:0101`: pass --skip-mon-network to configure it later
```