Installation issues for laravel 8

[theefreelancer]

This comes after typing the command composer install
Below is the error in the console or terminal;
PHP Fatal error: During inheritance of ArrayAccess: Uncaught ErrorException: Return type of Illuminate\Support\Collection::offsetExists($key) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /opt/lampp/htdocs/OpenSource/Employee-management-system-in-laravel/vendor/laravel/framework/src/Illuminate/Support/Collection.php:1941
Stack trace:
Below is my composer.json file
{
"name": "laravel/laravel",
"type": "project",
"description": "The Laravel Framework.",
"keywords": [
"framework",
"laravel"
],
"license": "MIT",
"require": {
"php": "^7.1.3|^8.1",
"brian2694/laravel-toastr": "^5.5",
"fideloper/proxy": "^4.0",
"laravel/framework": "5.8.*",
"laravel/tinker": "^1.0",
"maddhatter/laravel-fullcalendar": "^1.3"
},
"require-dev": {
"beyondcode/laravel-dump-server": "^1.0",
"filp/whoops": "^2.0",
"fzaninotto/faker": "^1.4",
"mockery/mockery": "^1.0",
"nunomaduro/collision": "^2.0",
"phpunit/phpunit": "^7.5"
},
"config": {
"optimize-autoloader": true,
"preferred-install": "dist",
"sort-packages": true
},
"extra": {
"laravel": {
"dont-discover": []
}
},
"autoload": {
"psr-4": {
"App\": "app/"
},
"classmap": [
"database/seeds",
"database/factories"
]
},
"autoload-dev": {
"psr-4": {
"Tests\": "tests/"
}
},
"minimum-stability": "dev",
"prefer-stable": true,
"scripts": {
"post-autoload-dump": [
"Illuminate\Foundation\ComposerScripts::postAutoloadDump",
"@php artisan package:discover --ansi"
],
"post-root-package-install": [
"@php -r "file_exists('.env') || copy('.env.example', '.env');""
],
"post-create-project-cmd": [
"@php artisan key:generate --ansi"
]
}
}
PS; PHP VERSION: 8.1
Laravel version 9