Update GATT characteristics code to match modern ESPHome properties.

[KittyFort]

The current source code doesn't compile against a recent ESPHome build (v2023.9.0). Some small changes will get it compiling and functioning.

• direct parent_ references were replaced with parent()
• direct gattc_if references were replaced with get_gattc_if()
• direct remote_bda references were replaced with get_remote_bda()

[mheistermann]

Thank you very much!