[Heartbeat] Setuid to regular user / lower capabilities when possible

[andrewvc]

Heartbeat should run with lower capabilities than root where possible for security reasons and for greater compatibility with node, which doesn't like running as root. See #27648 for more info.

[andrewvc]

Resolved via #27878

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[shahzad31]

Post FF Testing Looks good

verified that it's not using a root user to run heartbeat in the docker container

sh-4.2$ ps aux | grep heartbeat
elastic+      97  0.9  1.6 1706532 101412 ?      Sl   14:52   0:00 /usr/share/elastic-agent/data/elastic-agent-5ad13f/install/heartbeat-7.16.0-SNAPSHOT-linux-x86_64/heartbeat -E setup.ilm.enabled=false -E setup.template.enabled=false -E management.enabled=true -E logging.level=debug -E logging.level=info -E http.enabled=true -E http.host=unix:///usr/share/elastic-agent/data/tmp/default/heartbeat/heartbeat.sock -E logging.json=true -E logging.ecs=true -E logging.files.path=/usr/share/elastic-agent/data/logs/default -E logging.files.name=heartbeat-json.log -E logging.files.keepfiles=7 -E logging.files.permission=0640 -E logging.files.interval=1h -E path.data=/usr/share/elastic-agent/data/run/default/heartbeat--7.16.0-SNAPSHOT
elastic+    1267  0.0  0.0   9108   788 pts/0    S+   14:54   0:00 grep heartbeat