Support psr/http-message:^2.0

[crabmusket]

See https://www.php-fig.org/psr/psr-7/meta/#72-type-additions

It looks like this package is already compatible with the new type annotations: https://github.com/HubSpot/hubspot-php/commit/ef7e73e5e872edde487d7587b2cd9ecc2297832b

However, composer.json only declares compatibility with version 1.1.