fix: dev: Fix invalid pointer release in JSON statistics-channel response

Each response served on a JSON statistics endpoint released the wrong
pointer to the JSON library after the response was sent: the response
body string instead of the JSON document.  With the current responses
this does not crash named in practice, but the call is incorrect and
can in principle corrupt memory.  XML responses are not affected.

Closes #6024

Merge branch '6024-statschannel-json-response-invalid-free' into 'main'

See merge request isc-projects/bind9!12068