Add KillSignal=SIGQUIT to default systemd service file

Description

The current systemd .service file generated by gitlab-runner install does not set KillSignal, so systemd will send the default SIGTERM and immediately kill the runner and all child processes.

Proposal

It should do a graceful shutdown instead. I would propose the following settings:

[Service]
KillSignal=SIGQUIT
# SIGQUIT goes only to the main process, SIGKILL goes to all processes in the cgroup
KillMode=mixed

Links to related issues and merge requests / references

Tomasz outlined how to properly use KillSignal to initiate a graceful shutdown at #27093 (comment 793812818).

This was done for docker images in !1427 (merged). It was proposed for systemd services in !2409 (closed) but rejected.