Replace GCP deployment-manager scripts with infrastructure-manager

[orouz]

Motivation
deployment-manager will be deprecated in December 31, 2025 March 31, 2026 (extended) and replaced with infrastructure-manager

Definition of done

• Explore the new infra-manager service and replace code in deploy/deployment-manager in order to support automatic deployments on GCP
• Provide a migration script for existing resources deployed with the deployment-manager

[amirbenun]

Apart of the deploy.sh that deploys elastic agent on a VM, our deployment manager offering also provides deploy_service_account.sh that only deploys the Service Account and the user can paste the credentials json back on kibana.
This option of deploying the service account is only supported on GCP where we don't yet have Cloud Connectors.
Assuming that Cloud Connectors for GCP will be available on the next release 9.4, is this flow need to be supported as part of the migration to infra manager? Or maybe we can drop it as we will provide the full Cloud Connector experience.
CC @olegsu @smriti0321

Generate service account screenshot:

[olegsu]

Update:
After discussion, we will leave both options

[amirbenun]

Yes
I have created two PRs:
#3835
#3776

[olegsu]

Adding two items to track

• Update the link in elastic/integration
• Update public documentation with instruction to migrate