Bump golang.org/x/net to 0.12.0
While not necessary to directly address CVE-2023-29406 (which should be
handled by using a patched version of Go when building), an
accompanying change to HTTP/2 error handling does impact agent code.
See https://go-review.googlesource.com/c/net/+/506995 for the HTTP/2
change.
Bump this dependency across our submodules as well for the sake of
potential indirect consumers of `x/net/http`.
Manual backport of 84cbf09185ebfc59f9fcf486d1c4983ef129bf95.
