Resolves #12095 by WONTFIXing it.

This approach disables `writeToFile` as it allows arbitrary host
filesystem writes and is only a small quality of life improvement over
multiple `template` stanzas.

This approach has the significant downside of leaving people who have
altered their `template.function_denylist` *still vulnerable!* I added
an upgrade note, but we should have implemented the denylist as a
`map[string]bool` so that new funcs could be denied without overriding
custom configurations.

This PR also includes a bug fix that broke enabling all consul-template
funcs. We repeatedly failed to differentiate between a nil (unset)
denylist and an empty (allow all) one.