Android AdServices appears to be a core system app that is part of the Android Open Source Project (AOSP).
It is generally discouraged to change permissions on system apps as it can break basic app functionality. I don't believe it's possible to remove system apps without rooting your phone (another discouraged practice).
Obviously you are welcome to do whatever you want with your phone, but know that doing so may leave you on your own for support.
On a personal note, a great deal of security expertise has clearly gone into the design and execution of GrapheneOS. While I don't blindly trust any 3rd party, I do trust that if a core AOSP system app has been included in this project, it is done so with consideration. The same goes for the default permissions.