Replace deprecated DateTimeInterface::RFC7231 with literal format string

DateTimeInterface::RFC7231 is deprecated in PHP 8.5 because the format
hardcodes GMT timezone, ignoring the actual timezone of the instance.

Since gmdate() is used here (always GMT), the behavior is unchanged.

See: php/php-src#12989

Author: koriym

src/DevEtagSetter.php

@@ -7,7 +7,6 @@
 use BEAR\RepositoryModule\Annotation\HttpCache;
 use BEAR\Resource\Request;
 use BEAR\Resource\ResourceObject;
-use DateTimeInterface;
 use Override;
 
 use function gmdate;
@@ -30,7 +29,7 @@ public function __invoke(ResourceObject $ro, int|null $time = null, HttpCache|nu
         // This is useful for debugging purposes.
         // Usually, the ETag is a hash of the resource view or body.
         $ro->headers[Header::ETAG] = $uriEtag;
-        $ro->headers[Header::LAST_MODIFIED] = gmdate(DateTimeInterface::RFC7231, 0);
+        $ro->headers[Header::LAST_MODIFIED] = gmdate('D, d M Y H:i:s \G\M\T', 0);
         $this->setCacheDependency($ro);
     }
 

src/EtagSetter.php

@@ -7,7 +7,6 @@
 use BEAR\RepositoryModule\Annotation\HttpCache;
 use BEAR\Resource\Request;
 use BEAR\Resource\ResourceObject;
-use DateTimeInterface;
 use Override;
 
 use function assert;
@@ -38,7 +37,7 @@ public function __invoke(ResourceObject $ro, int|null $time = null, HttpCache|nu
 
         $etag =  $this->getEtag($ro, $httpCache);
         $ro->headers[Header::ETAG] = $etag;
-        $ro->headers[Header::LAST_MODIFIED] = gmdate(DateTimeInterface::RFC7231, $time);
+        $ro->headers[Header::LAST_MODIFIED] = gmdate('D, d M Y H:i:s \G\M\T', $time);
         $this->setCacheDependency($ro);
     }