doc: net: Update LwM2M block of overview document

Re-word the support block for LwM2M:
- the supported interfaces
- change wording of supported LwM2M objects to include the "required"
  keyword instead of explicitly listing them
- mention several IPSO Smart Objects

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/16327



Signed-off-by: Michael Scott <mike@foundries.io>