Feedback from testing operator hub integration releasing ECK 2.12.0

Feedback after testing operator hub integration using the [new process](https://github.com/elastic/k8s-dev/blob/main/release/operatorhub/README.md#validation) for [release 2.12](https://github.com/elastic/k8s-dev/issues/488).

Nit: the Redhat API key can also be retrieved from Vault, instead of Keybase.
```
vault kv get -field=api-key secret/ci/elastic-cloud-on-k8s/operatorhub-release-redhat
```

`generate-bundle` fails:
```
> make generate-bundle
opm alpha bundle build --directory tmp/certified-operators/2.12.0-bc2 \
--tag eck-operator-bundle:2.12.0-bc2 --output-dir tmp/certified-operators-bundle
Error: The directory /Users/krkr/../operatorhub/tmp/certified-operators/2.12.0-bc2 contains no yaml files
```

I'm not sure that `opm alpha bundle build` is still something?

`validate-bundle` succeeds with warnings:
```sh
> m validate-bundle
operator-sdk bundle validate tmp/certified-operators/2.12.0-bc2 --select-optional name=operatorhub
WARN[0000] Warning: Value autoscaling.k8s.elastic.co/v1alpha1, Kind=ElasticsearchAutoscaler: provided API should have an example annotation
WARN[0000] Warning: Value stackconfigpolicy.k8s.elastic.co/v1alpha1, Kind=StackConfigPolicy: provided API should have an example annotation
WARN[0000] Warning: Value elasticsearch-eck-operator-certified.v2.12.0-bc2: this bundle is using APIs which were deprecated and removed in v1.25. More info: https://kubernetes.io/docs/reference/using-api/deprecation-guide/#v1-25. Migrate the API(s) for poddisruptionbudgets: (["ClusterServiceVersion.Spec.InstallStrategy.StrategySpec.Permissions[0].Rules[6]"])
WARN[0000] Warning: Value : The "operatorhub" validator is deprecated; for equivalent validation use "operatorhub/v2", "standardcapabilities" and "standardcategories" validators
INFO[0000] All validation tests have completed successfully
```

`provided API should have an example annotation` is harmless.

Not sure for `WARN[0000] Warning: Value elasticsearch-eck-operator-certified.v2.12.0-bc2: this bundle is using APIs which were deprecated and removed in v1.25. More info: https://kubernetes.io/docs/reference/using-api/deprecation-guide/#v1-25. Migrate the API(s) for poddisruptionbudgets: (["ClusterServiceVersion.Spec.InstallStrategy.StrategySpec.Permissions[0].Rules[6]"])`.

It looks like this issue:
- https://github.com/operator-framework/operator-sdk/issues/6237.

`make test-deploy check-test-deploy apply-quickstart` succeeds after a small change to build the catalog:

```diff
diff --git a/release/operatorhub/Makefile b/release/operatorhub/Makefile
index 3edfb5a..087d7c6 100644
--- a/release/operatorhub/Makefile
+++ b/release/operatorhub/Makefile
@@ -53,7 +54,8 @@ build-catalog:

sed "s|@ECK_VERSION|${ECK_VERSION}|" olm/bundle.Dockerfile > olm/Dockerfile
docker build -t docker.io/${DOCKER_USER}/operators:${ECK_VERSION} -f olm/Dockerfile .
- opm index add --bundles docker.io/${DOCKER_USER}/operators:eck-operator-${ECK_VERSION} --tag ${DOCKER_USER}/operators:eck-operator-catalog-${ECK_VERSION} -c docker
+ docker push docker.io/${DOCKER_USER}/operators:${ECK_VERSION}
+ opm index add --bundles docker.io/${DOCKER_USER}/operators:${ECK_VERSION} --tag ${DOCKER_USER}/operators:eck-operator-catalog-${ECK_VERSION} -c docker
```