Update integ tests doc after moving to ESS staging

[pchila]

What does this PR do?

Updates documentation with the correct url for creating an API key.

Why is it important?

We want to make full use of staging ESS to improve stability of our integration tests runs

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/fragments using the changelog tool
• I have added an integration test or an E2E test

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates Move integration test to ess staging #3535

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• 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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• Start Time: 2023-10-05T17:14:40.914+0000

• Duration: 7 min 11 sec

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