[8.17](backport #6413) [ESS Testing] Update ESS region and deployment template ID

[mergify]

What does this PR do?

This PR updates the GCP region and ESS deployment template ID in the Terraform variables file to values that are currently acceptable in ESS, as per https://www.elastic.co/guide/en/cloud/current/ec-regions-templates-instances.html.

Why is it important?

So ESS deployments can be spun for testing Elastic Agent changes in ESS, as per https://github.com/elastic/elastic-agent?tab=readme-ov-file#testing-on-elastic-cloud

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

Disruptive User Impact

None, as this change only impacts Elastic Agent developers.

How to test this PR locally

1. Run EC_API_KEY=your_api_key make apply, replacing your_api_key with an API key from your ESS account.
2. You may see failures about the Elastic Agent version and the command may not succeed, but you should not see failures like api error: deployments.elasticsearch_using_legacy_dt: The referenced deployment template in elasticsearch plan [...] is no longer available.

Related issues

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 #6413 done by [Mergify](https://mergify.com).

[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