Fix EK stack deployment on kubernetes

[juanluisvaladas]

What does this PR do?

This PR fixes the EK stack deployment on kubernetes by changing kibana's username from elastic to kibana_system.

This was broken in elastic/kibana#122722 and it provoked kibana not to start with error:

$ k logs -p kibana-76f54554c9-8rsbq
[2022-09-22T10:48:41.271+00:00][INFO ][plugins-service] Plugin "metricsEntities" is disabled.
[2022-09-22T10:48:41.307+00:00][FATAL][root] Error: [config validation of [elasticsearch].username]: value of "elastic" is forbidden. This is a superuser account that cannot write to system indices that Kibana needs to function. Use a service account token instead. Learn more: https://www.elastic.co/guide/en/elasticsearch/reference/8.0/service-accounts.html
    at ensureValidConfiguration (/usr/share/kibana/src/core/server/config/ensure_valid_configuration.js:25:11)
    at Server.preboot (/usr/share/kibana/src/core/server/server.js:160:5)
    at Root.preboot (/usr/share/kibana/src/core/server/root/index.js:48:14)
    at bootstrap (/usr/share/kibana/src/core/server/bootstrap.js:99:9)
    at Command.<anonymous> (/usr/share/kibana/src/cli/serve/serve.js:216:5)

 FATAL  Error: [config validation of [elasticsearch].username]: value of "elastic" is forbidden. This is a superuser account that cannot write to system indices that Kibana needs to function. Use a service account token instead. Learn more: https://www.elastic.co/guide/en/elasticsearch/reference/8.0/service-accounts.html

How to test this PR locally

1- Make sure that you have pulled the latest docker.elastic.co/kibana/kibana:8.0.0-SNAPSHOT. Most certainly this tag used to point to a different image which used to work before.
2- Follow the instructions of https://github.com/elastic/beats/blob/main/metricbeat/module/kubernetes/_meta/test/docs/README.md.
The only changes are in one file so running kubectl apply -f metricbeat/module/kubernetes/_meta/test/docs/01_playground/ek_stack.yaml before and after will show the difference.

Related issues

elastic/kibana#123113
elastic/kibana#122722

[cla-checker-service]

💚 CLA has been signed

[botelastic]

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @juanluisvaladas? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-09-22T15:37:53.514+0000

• Duration: 5 min 47 sec

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[juanluisvaladas]

Hi approvers, I don't think I have permissions to add labels so I guess you'll need to do it for me.

Finally at this point it would probably be a good idea to update the version to 8.4. Can I bump it in another commit of this PR or should I make a new one?