CachedKeySet should use an uppercase GET method with PSR HttpClient

[noemi-salaun]

In the class CachedKeySet, the http request is created with a lowercase get method.
Standard HTTP method defined in the RFC are all uppercase and PSR HttpClient are explicitly asked not to modify the case because methods are case sensitive.

See https://www.php-fig.org/psr/psr-7/

 * While HTTP method names are typically all uppercase characters, HTTP
 * method names are case-sensitive and thus implementations SHOULD NOT
 * modify the given string.

Some HttpClient implementation, like the one from Symfony prevent the use of lowercase method name, so we cannot use them as ClientInterface for the CachedKeySet