[The current minimum supported Go version is now
1.23](62ec29fd9b/go.mod (L3)).
`context.AfterFunc` is available for all of grpc-go's latest version
users. Thus we can do this pending TODO.
`context.AfterFunc` would invoke the given function for both _immediate_
context cancelation and timer-based context cancelation (`WithTimeout`,
`WithDeadline`). So I think this change is safe.
RELEASE NOTES: N/A