Refactor workflows and remove unused environment variables

**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