chore: Set AES128GCM as default encoding

[swiftsjh02]

Summary

This PR changes the default encryption encoding used in the PushService class from AESGCM to AES128GCM.

Rationale

While AESGCM is a newer encoding method, some modern browsers (e.g., Chrome, Edge) currently do not properly trigger service workers when receiving push notifications encoded with aesgcm.

By contrast, aes128gcm is widely supported and works reliably across all major browsers. This change ensures more consistent and reliable push notification delivery.

Changes

• Updated the send(Notification) method in PushService to use Encoding.AES128GCM by default.

Reference

• MDN Web Push documentation
• Web Push encryption reference

##Notes

• This is My First Contribution , Give any Opinion And Advice Freely.

[martijndwars]

Thanks!