Use a non-deprecated date constant for phing.startTime property

Author: mrook

src/Phing/Phing.php

@@ -47,7 +47,6 @@
 use SebastianBergmann\Version;
 use Symfony\Component\Console\Output\ConsoleOutput;
 use Throwable;
-
 use function array_filter;
 use function array_map;
 use function array_reduce;
@@ -58,8 +57,6 @@
 use function strlen;
 use function strval;
 use function trim;
-
-use const DATE_RFC7231;
 use const PHP_EOL;
 
 /**
@@ -1669,7 +1666,7 @@ private static function setSystemConstants(): void
         self::setProperty(self::PHP_VERSION, PHP_VERSION);
         self::setProperty('php.tmpdir', sys_get_temp_dir());
         self::setProperty('application.startdir', getcwd());
-        self::setProperty('phing.startTime', gmdate(DATE_RFC7231));
+        self::setProperty('phing.startTime', gmdate(\DateTimeInterface::RFC2822));
 
         // try to detect machine dependent information
         $sysInfo = [];