BREAKING CHANGE: fix multiple issues with google provider v4

The main issue this MR addresses is the fact that we cannot support both google provider v3 and v4 due to the following options being deprecated in v4:

1. The google_container_cluster.workload_identity_config block no longer allows the identity_namespace option, this has been renamed to workload_pool.
2. The google_container_cluster.workload_metadata_config block no longer allows the node_metadata option, this has been renamed to mode.

NOTE - Due to this issue I've added a section to the README to advise on restricting the google provider versions when deploying an AutoPilot cluster. I've decided against restricting the version in this module itself as the issue does not affect standard GKE deployments, which should continue to use the latest versions of the google providers where possible.

There is also a commit to enable using custom service accounts for the GKE nodes, rather than the default compute service account. However, the autopilot cluster bit is currently commented out with a link to an open issue which is blocking us actually using it at the moment. It is tried and tested for non-autopilot clusters.

Finally, there is a commit to remove the usage of the deprecated hashicorp/template provider and instead use the built-in templatefile function.

Once merged, a new major version tag and branch will be created due to the breaking nature of these changes.

Closes #32 (closed), #31 (closed)