feat: PHP 8.1 support

This patch provides PHP 8.1 support, via the following changes:

- Adds `~8.1.0` to the list of allowed PHP versions.
- Marks a conflict with symfony/console versions prior to 5.3.11 (first version supporting PHP 8.1)
- Replaces usage of `gmstrftime()` with a combination of casting timestamps to `DateTimeImmutable` objects with a GMT timezone, and using `IntlDateFormatter::formatObject()` with an equivalent date/time format to what was used with `gmstrftime()`.
- Adds `#[\ReturnTimeWillChange]` attributes to classes implementing `JsonSerialize` to ensure they remain both backwards and forwards compatible.

Signed-off-by: Matthew Weier O'Phinney <matthew@weierophinney.net>

.laminas-ci.json

@@ -2,5 +2,8 @@
     "extensions": [
         "inotify",
         "swoole"
-    ]
+    ],
+    "ignore_php_platform_requirements": {
+        "8.1": true
+    }
 }

composer.json

@@ -28,7 +28,7 @@
     "extra": {
     },
     "require": {
-        "php": "^7.4 || ~8.0.0",
+        "php": "^7.4 || ~8.0.0 || ~8.1.0",
         "composer/package-versions-deprecated": "^1.11",
         "dflydev/fig-cookies": "^2.0.1 || ^3.0",
         "laminas/laminas-cli": "^0.1.5 || ^1.0",
@@ -48,7 +48,7 @@
         "filp/whoops": "^2.1",
         "laminas/laminas-coding-standard": "~2.2.0",
         "laminas/laminas-servicemanager": "^3.3",
-        "phpunit/phpunit": "^9.3.10",
+        "phpunit/phpunit": "^9.5.5",
         "psalm/plugin-phpunit": "^0.13.0",
         "swoole/ide-helper": "^4.5.5",
         "vimeo/psalm": "^4.0"
@@ -79,7 +79,8 @@
         "test": "phpunit --colors=always",
         "test-coverage": "phpunit --colors=always --coverage-clover clover.xml"
     },
-    "replace": {
-        "zendframework/zend-expressive-swoole": "^2.5.0"
+    "conflict": {
+        "symfony/console": "<5.3.11",
+        "zendframework/zend-expressive-swoole": "*"
     }
 }