Bump PHP version requirement from 5.2.3 to 7.2.25

Summary:
Supporting PHP 8.4 requires syntax introduced in PHP 7.1.
Thus require PHP version 7.2.25 released on 21 November 2019.
Phorge previously required version 5.2.3 released on 31 May 2007.

Closes T15047

Test Plan: None.

Reviewers: O1 Blessed Committers, Sten, avivey

Reviewed By: O1 Blessed Committers, Sten, avivey

Subscribers: Sten, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15047

Differential Revision: https://we.phorge.it/D25815

src/docs/user/installation_guide.diviner

@@ -80,7 +80,7 @@ You will also need:
   - **MySQL**: You need MySQL. We strongly recommend MySQL 5.5 or newer.
     You will need a server with multiple databases.
   - **PHP**: You need a PHP engine:
-    - PHP 7 - 7.2 or newer.
+    - PHP 7 - 7.2.25 or newer.
     - PHP 8 - 8.0 or newer.
   - **git**: You need git 2.5.0 or newer on the server.
     No particular version is needed on your clients.

support/startup/PhabricatorStartup.php

@@ -535,7 +535,7 @@ final class PhabricatorStartup {
    * @task validation
    */
   private static function verifyPHP() {
-    $required_version = '5.2.3';
+    $required_version = '7.2.25';
     if (version_compare(PHP_VERSION, $required_version) < 0) {
       self::didFatal(
         "You are running PHP version '".PHP_VERSION."', which is older than ".