Remove use of deprecated DATE_RFC7231 constant

Author: paulbalandan

src/Nexus/Cookie/Cookie.php

@@ -184,7 +184,7 @@ public function __toString(): string
 
             if (0 !== $this->expires) {
                 $cookieParts[] = \sprintf('Max-Age=%d', $this->getMaxAge());
-                $cookieParts[] = \sprintf('Expires=%s', gmdate(DATE_RFC7231, $this->expires));
+                $cookieParts[] = \sprintf('Expires=%s', gmdate(self::EXPIRES_FORMAT, $this->expires));
             }
         }
 

src/Nexus/Cookie/CookieInterface.php

@@ -18,6 +18,11 @@
  */
 interface CookieInterface extends \Stringable
 {
+    /**
+     * RFC 7231 compliant date format for cookie expiration.
+     */
+    public const EXPIRES_FORMAT = 'D, d M Y H:i:s T';
+
     /**
      * Cookies will be sent in all contexts, i.e in responses to both
      * first-party and cross-origin requests. If `SameSite=None` is set,