SUP-9491- check warnings in lib-wallet-php-client

[JustasJankauskas-Paysera]

Added:

• PHP 8.x and PHP 9.x compatibility

Changed:

• Updated minimum PHP version from >=5.5 to >=7.1 (required for nullable types)
• Updated PHPUnit from 4.8 to 9.6
• Updated all test files to PHPUnit 9 syntax
• Added ext-mbstring as dev dependency for PHPUnit 9

Fixed:

• Fixed implicit nullable parameter deprecation warnings in PHP 8.1+
• Fixed EventDispatcher::dispatch() nullable parameter
• Fixed WalletClient::finalizePayment() nullable parameter
• Fixed WalletClient::getWalletStatements() nullable parameter
• Fixed WalletClient::getLocations() nullable parameter
• Fixed TokenRelatedWalletClient::getCurrentWalletStatements() nullable parameter
• Fixed LocationManager::isLocationOpen() nullable parameter
• Fixed Router::resolveEndpointPath() to properly handle null path parameter
• Fixed EndpointSetter::onBeforeRequest() substr() null safety
• Fixed missing return type declarations in EventSubscriber interface and implementations
• Fixed all PHPUnit deprecations in test suite

[scruoge]

PHP 9.x compatibility ??? o_O