Similar to our work with the migration to separate modules for the
`pkg/runtime` and `pkg/types` packages, we can do the same for the
middleware.

This allows us to iterate over these quicker than the underlying code
generator and gives us the opportunity to introduce breaking changes
more easily if appropriate.

These won't be removed until the next major version, but we can mark
them as deprecated to nudge folks to move away from them.

To nudge people to move along to the new versions in their own repos.

Instead of Dependabot, as it gives us more control.

As part of replacing `testutil` with a separate library, we're removing
this convenience method, as we can provide a full drop-in replacement
with `GoWithHTTPHandler`.

It appears that GoDoc does not parse these deprecation warnings as a
deprecation unless there is an explicit blank comment between lines, or
it is the start of a doc comment.

So they're more actionable, as the previous setup didn't quite make it
possible to link directly through to replacing symbol.