Symfony 4 PHP Application deployed to GAE fails with error 'Attempted to load class "Kernel" from namespace "App"' [closed]

GAE appers not to be able to load classes defined with autoloaded configuration in a PHP application using the Symfony 4 Framework.

Spinning up a simple application to deploy to GAE php73 runtime outputs the error 'Attempted to load class "Kernel" from namespace "App"'

Kernel in the above instane extends BaseKernel and it is used in index.php entry file for GAE PHP

<?php

use App\Kernel;
use Symfony\Component\Debug\Debug;
use Symfony\Component\HttpFoundation\Request;

require dirname(__DIR__).'/config/bootstrap.php';

if ($_SERVER['APP_DEBUG']) {
    umask(0000);

    Debug::enable();
}

if ($trustedProxies = $_SERVER['TRUSTED_PROXIES'] ?? $_ENV['TRUSTED_PROXIES'] ?? false) {
    Request::setTrustedProxies(explode(',', $trustedProxies), Request::HEADER_X_FORWARDED_ALL ^ Request::HEADER_X_FORWARDED_HOST);
}

if ($trustedHosts = $_SERVER['TRUSTED_HOSTS'] ?? $_ENV['TRUSTED_HOSTS'] ?? false) {
    Request::setTrustedHosts([$trustedHosts]);
}

$kernel = new Kernel($_SERVER['APP_ENV'], (bool) $_SERVER['APP_DEBUG']);
$request = Request::createFromGlobals();
$response = $kernel->handle($request);
$response->send();
$kernel->terminate($request, $response);

Application works as expected locally showing the expected LuckyNumber output from ApiController in image below.

Contents of composer.json

{
    "type": "project",
    "license": "proprietary",
    "require": {
        "php": ">=7.1.3",
        "ext-ctype": "*",
        "ext-iconv": "*",
        "symfony/console": "4.3.*",
        "symfony/dotenv": "4.3.*",
        "symfony/flex": "^1.3.1",
        "symfony/framework-bundle": "4.3.*",
        "symfony/yaml": "4.3.*"
    },
    "require-dev": {
        "symfony/web-server-bundle": "4.3.*"
    },
    "config": {
        "preferred-install": {
            "*": "dist"
        },
        "sort-packages": true
    },
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "App\\Tests\\": "tests/"
        }
    },
    "replace": {
        "paragonie/random_compat": "2.*",
        "symfony/polyfill-ctype": "*",
        "symfony/polyfill-iconv": "*",
        "symfony/polyfill-php71": "*",
        "symfony/polyfill-php70": "*",
        "symfony/polyfill-php56": "*"
    },
    "scripts": {
        "auto-scripts": {
            "cache:clear": "symfony-cmd",
            "assets:install %PUBLIC_DIR%": "symfony-cmd"
        },
        "post-install-cmd": [
            "@auto-scripts"
        ],
        "post-update-cmd": [
            "@auto-scripts"
        ]
    },
    "conflict": {
        "symfony/symfony": "*"
    },
    "extra": {
        "symfony": {
            "allow-contrib": false,
            "require": "4.3.*"
        }
    }
}

tried adding \App\ to autoload psr-4 configuration as below, but one GAE I still get error that Kernel used in index.php can't be found.

"autoload": {
        "psr-4": {
            "\\App\\": "src/"
        }
    },