tini should be used with -s argument

[orfeas-k]

Bug Description

Due to charms having pebble as PID 1, we should deviate from upstream and use tini with -s as instructed in the docs and also in the warning we get, when using it without:

2024-07-24T07:38:24.073Z [kubeflow-dashboard] [WARN  tini (15)] Tini is not running as PID 1 and isn't registered as a child subreaper.
2024-07-24T07:38:24.074Z [kubeflow-dashboard] Zombie processes will not be re-parented to Tini, so zombie reaping won't work.
2024-07-24T07:38:24.074Z [kubeflow-dashboard] To fix the problem, use the -s option or set the environment variable TINI_SUBREAPER to register Tini as a child subreaper, or run Tini as PID 1.

Thus th ecommand will be:

self._service = "/sbin/tini -s -- npm start"

To Reproduce

N/A

Environment

N/a

Relevant Log Output

n/a

Additional Context

No response

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-6050.

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[orfeas-k]

From our discussion with rockcraft, it turns out that since we have pebble, we don't need to use tini. Thus, instead of adding an -s argument, we can just remove altogether.

Thus, we need to:

1. Remove relevant comment (in main and track 1.9).
2. Add context somewhere appropriate. Contributing.md file was suggested during the bug triage meeting but I don't think it's an appropriate place after all for adding an implementation detail. Modifying the comment above (next to the command) to link to this issue's comment could provide enought context too.