Add logging of "oldTLS" bit (letsencrypt#6008)

That causes the VA to emit ValidationRecords with the OldTLS bit set if
it observes a redirect to HTTPS that negotiates TLS < 1.2.

I've manually tested but there is not yet an integration test. I need
to make a parallel change in challtestsrv and then incorporate here.

core/objects.go

@@ -169,6 +169,11 @@ type ValidationRecord struct {
 	//   ...
 	// }
 	AddressesTried []net.IP `json:"addressesTried,omitempty"`
+
+	// OldTLS is true if any request in the validation chain used HTTPS and negotiated
+	// a TLS version lower than 1.2.
+	// TODO(#6011): Remove once TLS 1.0 and 1.1 support is gone.
+	OldTLS bool `json:"oldTLS,omitempty"`
 }
 
 func looksLikeKeyAuthorization(str string) error {

va/http.go

@@ -494,6 +494,7 @@ func (va *ValidationAuthorityImpl) processHTTPValidation(
 	// addresses explicitly, not following redirects to ports != [80,443], etc)
 	records := []core.ValidationRecord{baseRecord}
 	numRedirects := 0
+	var oldTLS bool
 	processRedirect := func(req *http.Request, via []*http.Request) error {
 		va.log.Debugf("processing a HTTP redirect from the server to %q", req.URL.String())
 		// Only process up to maxRedirect redirects
@@ -503,6 +504,11 @@ func (va *ValidationAuthorityImpl) processHTTPValidation(
 		numRedirects++
 		va.metrics.http01Redirects.Inc()
 
+		// TODO(#6011): Remove once TLS 1.0 and 1.1 support is gone.
+		if req.Response.TLS != nil && req.Response.TLS.Version < tls.VersionTLS12 {
+			oldTLS = true
+		}
+
 		// If the response contains an HTTP 303 or any other forbidden redirect,
 		// do not follow it. The four allowed redirect status codes are defined
 		// explicitly in BRs Section 3.2.2.4.19. Although the go stdlib currently
@@ -618,6 +624,15 @@ func (va *ValidationAuthorityImpl) processHTTPValidation(
 			records[len(records)-1].URL, records[len(records)-1].AddressUsed, httpResponse.StatusCode)
 	}
 
+	// TODO(#6011): Remove once TLS 1.0 and 1.1 support is gone.
+	if httpResponse.TLS != nil && httpResponse.TLS.Version < tls.VersionTLS12 {
+		oldTLS = true
+	}
+
+	if oldTLS {
+		records[len(records)-1].OldTLS = true
+	}
+
 	// At this point we've made a successful request (be it from a retry or
 	// otherwise) and can read and process the response body.
 	body, err := ioutil.ReadAll(&io.LimitedReader{R: httpResponse.Body, N: maxResponseSize})