Update absl dependency to https://github.com/abseil/abseil-cpp/releases/tag/20250512.1
As part of this upgrade, remove gRPC's usage of internal absl helper functions.
A similar fix is needed for OpenCensus-Cpp, but since that repo is archived, I am choosing to add in a patch. The aim is to delete our OpenCensus plugin so that we don't have to maintain this forever as per https://github.com/grpc/proposal/pull/496.
Closes#39571
PiperOrigin-RevId: 777770232
