Code Modernization: Replace `strftime()` and `gmstrftime()` usage in unit tests.

Since PHP 8.1, the `strftime()` and `gmstrftime()` functions are deprecated:

> The `strftime()` and `gmstrftime()` functions exhibit similar issues as `strptime()`, in that the formats they support, as well as their behavior, is platform-dependent. Unlike `strptime()`, these functions are available on Windows, though with a different feature set than on Linux. Musl-based distributions like Alpine do not support timezone-related format specifiers correctly. These functions are also locale-based, and as such may exhibit thread-safety issues.
>
> `date()` or `DateTime::format()` provide portable alternatives, and `IntlDateFormatter::format()` provides a more sophisticated, localization-aware alternative.

Reference: [https://wiki.php.net/rfc/deprecations_php_8_1#strftime_and_gmstrftime PHP RFC: Deprecations for PHP 8.1: strftime() and gmstrftime()]

> The `strftime()` and `gmstrftime()` functions have been deprecated in favor of
> `date()/DateTime::format()` (for locale-independent formatting) or
> `IntlDateFormatter::format()` (for locale-dependent formatting).

Reference: [1cf4fb739f/UPGRADING (L379-L381) PHP 8.1 Upgrade Notes].

Aside from one instance in SimplePie, the `strftime()` and `gmstrftime()` functions are only used within the test suite of WordPress to create formatted timestamps.

As the function is used in test code, this leads to test warnings like this on PHP 8.1:
{{{
Deprecated: Function strftime() is deprecated in path/to/tests/phpunit/tests/canonical/postStatus.php on line 37
}}}

These calls can all be safely converted to use a pattern along the lines of:
{{{#!php
<?php
date_format( date_create( 'time phrase or timestamp' ), $format )
}}}

Other references:
* [https://www.php.net/manual/en/function.strftime.php PHP Manual: strftime()] (for the old format string characters)
* [https://www.php.net/manual/en/datetime.format.php PHP Manual: DateTime::format()] (for the new format string characters)
* [https://www.php.net/manual/en/datetime.construct.php PHP Manual: DateTime::__construct()] (see Example 2 for a Unix timestamp code sample)

Props jrf, SergeyBiryukov.
Fixes #53897.
Built from https://develop.svn.wordpress.org/trunk@51587


git-svn-id: http://core.svn.wordpress.org/trunk@51198 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51586';
+$wp_version = '5.9-alpha-51587';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.