[integrations] - Replace gcs-mock-service in all existing integrations with pre-built docker image

[ShourieG]

Description: Currently a few integrations are using a custom built gcs-mock-service to perform gcs system tests. Our goal moving forward is to remove the redundant code of the mock service replacing it with a pre-built docker image. All future gcs system tests should also use the docker image directly.

Details on how to use the mock service as a docker image can be found here.

Current Integrations to be updated:

• akamai
• github
• google_cloud_storage
• netskope
• panw_cortex_xdr

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)