test: use CNPG released versions instead of testing release branches

[NiccoloFei]

When a CNPG version reaches its final release (e.g. 1.29.3 in the 1.29.x series), after it has been released it gets removed from CNPG pipelines, meaning that snapshots of the operator for this specific minor release won't be built anymore.
Being ghcr.io/cloudnative-pg/cloudnative-pg-testing images, existing snapshots will eventually be cleaned up by the automatic registry cleaner, until there will be no more snapshots available.
This results in a missing image while trying to install the latest release snapshots from the artifacts repo , like https://github.com/cloudnative-pg/artifacts/blob/release-1.29/manifests/operator-manifest.yaml - this manifest currently points to an image that doesn't exist, and won't be updated anymore going forward.

To avoid this issue, which would cause smoke tests to fail while bootstrapping the env using the PREVIOUS_RELEASE until a new CNPG minor release is done (1.31.0 in this case), when testing against CNPG releases we should instead target officially released versions.
We should test the latest released patch of the latest and previous supported minors.
This means that we'll have to start tracking CNPG_RELEASE_DEFAULT up to the patch version, since it's required to pull the target release manifest, e.g. https://github.com/cloudnative-pg/cloudnative-pg/blob/release-1.29/releases/cnpg-1.29.3.yaml

[ardentperf]

The logic to find the previous release is a little hairy but the overall approach looks good to me