[chore] Upgrade golangci-lint, ignore existing int overflow warnings

There is a new lint for unchecked int casts. Integer overflows are bad,
but the old code that triggers this lint seems to be perfectly fine.
Instead of disabling the lint entirely for new code as well, grandfather
in existing code.

internal/httpclient/client.go

@@ -340,14 +340,14 @@ func (c *Client) do(r *Request) (rsp *http.Response, retry bool, err error) {
 
 			if u, _ := strconv.ParseUint(after, 10, 32); u != 0 {
 				// An integer no. of backoff seconds was provided.
-				r.backoff = time.Duration(u) * time.Second
+				r.backoff = time.Duration(u) * time.Second //nolint:gosec
 			} else if at, _ := http.ParseTime(after); !at.Before(now) {
 				// An HTTP formatted future date-time was provided.
 				r.backoff = at.Sub(now)
 			}
 
 			// Don't let their provided backoff exceed our max.
-			if max := baseBackoff * time.Duration(c.retries); //
+			if max := baseBackoff * time.Duration(c.retries); //nolint:gosec
 			r.backoff > max {
 				r.backoff = max
 			}