net: lwm2m: json: use plain text formatter for float32/64

Current JSON formatting for float32/64 is broken in a similar way as
plain text.  Let's use the newly fixed logic for plain text to
generate the float32/64 values in the JSON string.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/16154



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