Pushy v0.15.1 - `location` push type and revised token handling

This is a minor release that adds support for the location push type introduced in iOS 15 (thank you, @fairEnough83) and fixes an issue with token handling that could lead to notifications getting rejected with TooManyProviderTokenUpdates under certain circumstances. This release also updates Pushy's dependency on Netty to use the latest version (4.1.74).