[Build] Set max PHP version to 8.0

This version does not support PHP 8.1 because it introduces a
breaking change. The next major version of this package will add
support for PHP 8.1.

Author: lindyhopchris

Diff for: CHANGELOG.md

@@ -3,6 +3,13 @@
 All notable changes to this project will be documented in this file. This project adheres to
 [Semantic Versioning](http://semver.org/) and [this changelog format](http://keepachangelog.com/).
 
+## Unreleased
+
+### Changed
+
+- The maximum PHP version is now 8.0. PHP 8.1 is not supported because it introduces a breaking change. The next major
+  version of this package will add support for PHP 8.1.
+
 ## [1.0.0] - 2021-07-31
 
 ### Added

Diff for: composer.json

@@ -23,7 +23,7 @@
         }
     ],
     "require": {
-        "php": "^7.4|^8.0",
+        "php": "^7.4|8.0.*",
         "ext-json": "*",
         "laravel-json-api/core": "^1.0.0",
         "laravel-json-api/eloquent": "^1.0.0",