chore: Deprecate old cookie value (#4336)

Older clis will need to be updated.
Modern clis cannot communicate with <8.15 coderd

codersdk/client.go

@@ -83,13 +83,6 @@ func (c *Client) Request(ctx context.Context, method, path string, body interfac
 	}
 	req.Header.Set(SessionCustomHeader, c.SessionToken)
 
-	// Delete this custom cookie set in November 2022. This is just to remain
-	// backwards compatible with older versions of Coder.
-	req.AddCookie(&http.Cookie{
-		Name:  "session_token",
-		Value: c.SessionToken,
-	})
-
 	if body != nil {
 		req.Header.Set("Content-Type", "application/json")
 	}