Code Modernization: Only set `auto_detect_line_endings` in PHP < 8.1.

Since PHP 8.1, the `auto_detect_line_endings` setting is deprecated:

> The `auto_detect_line_endings` ini setting modifies the behavior of `file()` and `fgets()` to support an isolated `\r` (as opposed to `\n` or `\r\n`) as a newline character. These newlines were used by “Classic” Mac OS, a system which has been discontinued in 2001, nearly two decades ago. Interoperability with such systems is no longer relevant.

Reference: [https://wiki.php.net/rfc/deprecations_php_8_1#auto_detect_line_endings_ini_setting PHP RFC: Deprecations for PHP 8.1: auto_detect_line_endings ini setting]

> The `auto_detect_line_endings` ini setting has been deprecated. If necessary, handle `\r` line breaks manually instead.

Reference: [1cf4fb739f/UPGRADING (L456-L457) PHP 8.1 Upgrade Notes].

This commit fixes the warning when running tests for the `PO` class:
{{{
Deprecated: auto_detect_line_endings is deprecated in /var/www/src/wp-includes/pomo/po.php on line 16
}}}

Follow-up to [10584], [51628].

See #53635.
Built from https://develop.svn.wordpress.org/trunk@51633


git-svn-id: http://core.svn.wordpress.org/trunk@51239 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/pomo/po.php

@@ -13,7 +13,10 @@ if ( ! defined( 'PO_MAX_LINE_LEN' ) ) {
 	define( 'PO_MAX_LINE_LEN', 79 );
 }
 
-ini_set( 'auto_detect_line_endings', 1 );
+// This setting has been deprecated in PHP 8.1.
+if ( PHP_VERSION_ID < 80100 ) {
+	ini_set( 'auto_detect_line_endings', 1 );
+}
 
 /**
  * Routines for working with PO files

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51632';
+$wp_version = '5.9-alpha-51633';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.