build(deps): bump github.com/gophercloud/gophercloud

Bumps [github.com/gophercloud/gophercloud](https://github.com/gophercloud/gophercloud) from 0.11.0 to 0.20.0.
- [Release notes](https://github.com/gophercloud/gophercloud/releases)
- [Changelog](https://github.com/gophercloud/gophercloud/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gophercloud/gophercloud/compare/v0.11.0...v0.20.0)

---
updated-dependencies:
- dependency-name: github.com/gophercloud/gophercloud
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

vendor/github.com/gophercloud/gophercloud/doc.go

@@ -106,5 +106,44 @@ intermediary processing on each page, you can use the AllPages method:
 This top-level package contains utility functions and data types that are used
 throughout the provider and service packages. Of particular note for end users
 are the AuthOptions and EndpointOpts structs.
+
+An example retry backoff function, which respects the 429 HTTP response code and a "Retry-After" header:
+
+	endpoint := "http://localhost:5000"
+	provider, err := openstack.NewClient(endpoint)
+	if err != nil {
+		panic(err)
+	}
+	provider.MaxBackoffRetries = 3 // max three retries
+	provider.RetryBackoffFunc = func(ctx context.Context, respErr *ErrUnexpectedResponseCode, e error, retries uint) error {
+		retryAfter := respErr.ResponseHeader.Get("Retry-After")
+		if retryAfter == "" {
+			return e
+		}
+
+		var sleep time.Duration
+
+		// Parse delay seconds or HTTP date
+		if v, err := strconv.ParseUint(retryAfter, 10, 32); err == nil {
+			sleep = time.Duration(v) * time.Second
+		} else if v, err := time.Parse(http.TimeFormat, retryAfter); err == nil {
+			sleep = time.Until(v)
+		} else {
+			return e
+		}
+
+		if ctx != nil {
+			select {
+			case <-time.After(sleep):
+			case <-ctx.Done():
+				return e
+			}
+		} else {
+			time.Sleep(sleep)
+		}
+
+		return nil
+	}
+
 */
 package gophercloud