Improve Collector process handling on Windows

[bernd]

We are currently seeing "invalid handle" errors in the process handling on Windows.

{"level":"info","ts":1777481496.4616299,"logger":"supervisor","caller":"supervisor/handle_own_logs.go:39","msg":"Restarting collector to apply own_logs changes"}
{"level":"warn","ts":1777481496.4616299,"logger":"supervisor","caller":"keen/keen.go:292","msg":"Failed to send shutdown signal, force-killing the agent process immediately","pid":9708,"error":"The handle is invalid."}
{"level":"warn","ts":1777481496.4616299,"logger":"supervisor","caller":"keen/keen.go:292","msg":"Failed to send shutdown signal, force-killing the agent process immediately","pid":9708,"error":"The handle is invalid."}
{"level":"warn","ts":1777481496.4616299,"logger":"supervisor","caller":"keen/keen.go:304","msg":"Couldn't kill process","pid":9708}

Investigate using context cancellation (exec.CommandContext) instead of signals on Windows.