Use Cmd.WaitDelay and Cmd.Cancel with CommandContext to simplify captive core termination

[tamirms]

In #3694 we introduced the following code which attempts to gracefully terminate captive core:

https://github.com/bartekn/go/blob/36251ee7536bd423a66803c0ead9825ccefd084d/ingest/ledgerbackend/stellar_core_runner.go#L357-L404

This code was taken from https://github.com/golang/go/blob/cacac8bdc5c93e7bc71df71981fdf32dded017bf/src/cmd/go/script_test.go#L1091-L1098 .

However, the Go 1.20 release added WaitDelay and Cancel properties to Cmd. These new features along with https://pkg.go.dev/os/exec#CommandContext can be used to simplify the termination logic for captive core (see golang/go@49abdbc for an example of how to use WaitDelay / Cancel).