net: lwm2m: Deprecate lwm2m_get/set_u64

Deprecate lwm2m_set_u64() and lwm2m_get_u64 as only
LWM2M_RES_TYPE_S64 exist. Unsigned variant is not defined.

Technically these might have worked OK, but it is undefined
what happens to large unsigned values when those are
converted to various payload formats (like CBOR) that might
decode numbers differently depending of their signedness.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>