Release: update user-agent, bundled CA, version number (#281)

* chore: update the user-agent we use

Part of the check-list at https://github.com/ooni/probe/issues/1369.

* chore: set version to 3.9.0

See https://github.com/ooni/probe/issues/1369

* chore: run go generate ./...

This is meant to update the bundled CA. We have heard of issues with
our bundled CA, but it seems there have been no changes upstream.

The website https://curl.se/docs/caextract.html still lists as the
last change the one done on Jan 19, 2021, which is the version of
the CA that we're currently bundling.

For the sake of continuing with the release process, I am going
to further investigate the CA once the release is done.

This chore is part of https://github.com/ooni/probe/issues/1369.

internal/engine/netx/gocertifi/certifi.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// 2021-03-10 12:20:35.042573349 +0100 CET m=+0.416591177
+// 2021-03-31 16:50:03.221493757 +0200 CEST m=+1.318280953
 // https://curl.haxx.se/ca/cacert.pem
 
 package gocertifi
@@ -3241,7 +3241,7 @@ kpzNNIaRkPpkUZ3+/uul9XXeifdy
 `
 
 // CACerts builds an X.509 certificate pool containing the
-// certificate bundle from https://curl.haxx.se/ca/cacert.pem fetch on 2021-03-10 12:20:35.042573349 +0100 CET m=+0.416591177.
+// certificate bundle from https://curl.haxx.se/ca/cacert.pem fetch on 2021-03-31 16:50:03.221493757 +0200 CEST m=+1.318280953.
 // Returns nil on error along with an appropriate error code.
 func CACerts() (*x509.CertPool, error) {
 	pool := x509.NewCertPool()