Migrating Concourse CI pipelines to GH Actions (rabbitmq#1208)

## Summary Of Changes

- Move the cluster operator to be a multiarch OCI built on AMD64 and ARM64, instead of just the former.
- Move existing Concourse CI infrastructure to GitHub Actions

## Additional Context

The pipeline now runs, on PRs:

- kubectl tests
- Unit tests
- Integration tests
- Tests that a multiarch OCI can be successfully built, but does not push it
- Runs system tests locally against kind, for multiple RabbitMQ Cluster versions
  - This excludes the volume expansion system tests
- Runs the documented examples against kind in 'dry-run' mode

On pushes to main:
- kubectl tests
- Unit tests
- Integration tests
- Builds a multiarch OCI of the operator (pushed to `rabbitmqoperator/cluster-operator` with a tag of the SHA256 and `edge`), and a manifest referencing that OCI
- Also builds a single-arch version of the image, for AMD64, for compatibility with internal tooling
- Runs system tests against GKE, for multiple RabbitMQ Cluster versions
- Runs the documented examples against GKE
- Tests if upgrading to the operator built earlier would cause a rolling restart of RabbitmqClusters

On creation of a tag in the repo:
- Same as push to main, except in addition:
  - OCI is tagged also with `latest` if the latest version, and the semver version of the release
  - Uploads manifests to a release in GitHub as a Draft release

N.B.: This PR removes the use of the rabbitmqoperator/cluster-operator-dev image, as it's now functionally replaced by the regular image with a SHA tag.

Merge pull request rabbitmq#1144 from rabbitmq/version-go-in-prom-tests

Explicitly install a known golang version.

Extract operator defaults to a separate func (rabbitmq#1068)

- to keep Reconciler shorter and more concise

Merge pull request rabbitmq#1040 from jiuker/fix_validateAndGetOwner

switch case change simple!

Merge pull request rabbitmq#1037 from rabbitmq/service-monitor-tls

Document how to set up tls verify for prometheus servicemonitor

Ensure client-go-originating logs are consistent with zap logger (rab…

…bitmq#983)

Merge pull request rabbitmq#969 from rabbitmq/dependabot/go_modules/k…

…8s.io/apimachinery-0.23.4

Correct quoting

Merge pull request rabbitmq#921 from rabbitmq/k-rmq-perf-test-improve…

…ments

kubectl rabbitmq perf-test improvements
- Replace `kubectl run` with kubernetes Job

This means that, if a finite number of messages are to be sent, then the kubernetes resources will be cleaned up automatically after completion.

Merge pull request rabbitmq#868 from rabbitmq/dependabot/go_modules/g…

…ithub.com/onsi/ginkgo-1.16.5