[8.18](backport #6719) [CI] Remove instances configurations from the integration test stack

[mergify]

What does this PR do?

Removes instance_configuration_id as @kuisathaverat suggested here.

Why is it important?

Instance types are selected by the deployment_template_id selected; the values of the instance types can change when the deployment_template_id. The use of fixed values could cause issues in deployments. Those instance types can be deleted, inappropriate for the stack version, expensive, consume more resources than needed, etc. We must use the instance types the Platform team selects for each deployment_template_id, they are the instance types tested in ECH.

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

• Closes https://github.com/elastic/ingest-dev/issues/4998

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

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This is an automatic backport of pull request #6719 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube