resolvconf: add //go:build directives to prevent downgrading to go1.16 language

Commit 8921897e3b941202d7d46143fa565008ca242205 introduced the uses of `clear()`,
which requires go1.21, but Go is downgrading this file to go1.16 when used in
other projects (due to us not yet being a go module);

    0.175 + xx-go build '-gcflags=' -ldflags '-X github.com/moby/buildkit/version.Version=b53a13e -X github.com/moby/buildkit/version.Revision=b53a13e4f5c8d7e82716615e0f23656893df89af -X github.com/moby/buildkit/version.Package=github.com/moby/buildkit -extldflags '"'"'-static'"'" -tags 'osusergo netgo static_build seccomp ' -o /usr/bin/buildkitd ./cmd/buildkitd
    181.8 # github.com/docker/docker/libnetwork/internal/resolvconf
    181.8 vendor/github.com/docker/docker/libnetwork/internal/resolvconf/resolvconf.go:509:2: clear requires go1.21 or later (-lang was set to go1.16; check go.mod)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/internal/resolvconf/resolvconf.go

@@ -1,3 +1,6 @@
+// FIXME(thaJeztah): remove once we are a module; the go:build directive prevents go from downgrading language version to go1.16:
+//go:build go1.21
+
 // Package resolvconf is used to generate a container's /etc/resolv.conf file.
 //
 // Constructor Load and Parse read a resolv.conf file from the filesystem or