[8.12](backport #38269) [Heartbeat] Add prctl dumpable flag reset after cap drop

[mergify]

Proposed commit message

Heartbeat lockdown measures are triggering /proc/io ownership rules, which end up not being accessible by the agent. This PR introduces a reset on PRCTL_SET_DUMPABLE flag to allow the agent to access heartbeat metrics.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

1. Enrol a local agent in a private location policy which contains at least one monitor.
2. Find PID for heartbeat process: docker exec -u root -it --container name-- ps -aef
3. Check permissions for /proc/--pid--/io:

//before
docker exec -u root -it vigorous_pike ls -al /proc/70/io
-r-------- 1 root root 0 Mar 12 12:59 /proc/70/io
//after
docker exec -u root -it exciting_spence ls -al /proc/57/io
-r-------- 1 elastic-agent elastic-agent 0 Mar 12 12:52 /proc/57/io
```<hr>This is an automatic backport of pull request #38269 done by [Mergify](https://mergify.com).

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• Start Time: 2024-03-18T13:24:43.920+0000

• Duration: 53 min 20 sec

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