Remove hard dependency for json=3.2.2

[eranb]

Current version of keycloak depended on json=2.6.2 that can cause issues especially on ruby ~>3.2 which have json=2.6.3 as it's default json gem.
If the user activate any gem that require "json", on ruby 3.2.2 -> json version 2.6.3 will be activated.
And if this happens before keycloak was loaded you will get activation exception after loading keycloak:
Unable to activate keycloak-3.2.2, because json-2.6.3 conflicts with json (= 2.6.2) (Gem::ConflictError)
See #49