Refactor workflows and remove unused environment variables

[gurevichdmitry]

Motivation In our current CI workflows, we have defined CI_ELASTIC_AGENT_DOCKER_TAG and CI_ELASTIC_AGENT_DOCKER_IMAGE variables specifically intended for EKS agent tests. However, as our current plans do not involve implementing agent tests for EKS, it's necessary to refactor our workflows and remove the usage of these variables.

Definition of done

• [ ] Review and modify all relevant CI workflows that use CI_ELASTIC_AGENT_DOCKER_TAG and CI_ELASTIC_AGENT_DOCKER_IMAGE variables.
• [ ] Remove any references to CI_ELASTIC_AGENT_DOCKER_TAG and CI_ELASTIC_AGENT_DOCKER_IMAGE from the workflows.
• [ ] Review and update the agent deploy YAML configuration used in our testing framework to ensure it no longer depends on CI_ELASTIC_AGENT_DOCKER_TAG and CI_ELASTIC_AGENT_DOCKER_IMAGE.
• [ ] Update any relevant documentation, READMEs, or other documentation resources that mention the deprecated variables to reflect the changes.
• [ ] Run the updated workflows to verify that they function correctly without the removed variables.

Out of scope NA

Related tasks/epics NA