Modify spec for cgroups v2 container ID parsing

[SylvainJuge]

Spec for capturing container ID when cgroups v2 is used #523

• May the instrumentation collect sensitive information, such as secrets or PII (ex. in headers)?
  • No
    • Why? container ID is not sensitive information and is already captured
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• If this spec adds a new dynamic config option, add it to central config. n/a

[basepi]

Add the /proc/self/cgroup to the json test case with the content: 0::/ so we can test fallback

[SylvainJuge]

Update after review feedback from @SergeyKleyman, we now have a single JSON for testing. The existing cgroup_parsing.json is currently only used by Java and .NET agents so I think it's fine to remove it in this PR.

Also, please note that adding/modifying json tests also require to modify .ci/updatecli.d/update-json-specs.yml in every agent repository. I'll check with the robots team to see if we could improve this in the future (see internal linked issue below this comment for details).

[Mpdreamz]

LGTM 👍 Like that its now contained to 1 test file too.