Fix Create Environment agent version for Serverless

[opauloh]

Summary of your changes

This fixes https://github.com/elastic/cloudbeat/issues/2546, by using the version inputted by the user in the Create Environment parameters instead of using the version from the env.ELK_VERSION.

While it is true that the version doesn't matter on Serverless in terms of Kibana and Elasticsearch, it does matter for deploying agents as the snapshots might be no longer available.

[mergify[bot]]

This pull request does not have a backport label. Could you fix it @opauloh? 🙏 To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit NOTE: backport-v8.x has been added to help with the transition to the new branch 8.x.

[eyalkraft]

Thanks Paulo! I'll try to deploy an env from this brach to validate

• https://github.com/elastic/cloudbeat/actions/runs/11048460122

[opauloh]

@gurevichdmitry it seems I can't merge this PR even after it was approved, could you merge it?