[gke] skip hard error when client misses permissions for some projects

[tigrato]

This PR fixes a behavior problem that results in GKE discovery to completely fail when the client misses permissions in some of the discovered projects.

If the client misses list permissions for projectID 1 but has the required permissions for projectID 2, the discovery service should continue with the projectID 2 discovery and skip the failing one.

Fixes #48101

Changelog: Fixed a bug in GKE auto-discovery where the process failed to discover any clusters if the identity lacked permissions for one or more detected GCP project IDs.

[ryanclark]

I feel like this is a nicer way of doing this

Suggested change

	if trace.IsAccessDenied(err) {
	a.Logger.WarnContext(ctx, "Access denied to list GKE clusters", "project_id", projectID, "location", a.Location)
	return nil, nil
	} else if err != nil {
	return nil, trace.Wrap(err)
	}
	if err != nil {
	if trace.IsAccessDenied(err) {
	a.Logger.WarnContext(ctx, "Access denied to list GKE clusters", "project_id", projectID, "location", a.Location)
	return nil, nil
	}
	return nil, trace.Wrap(err)
	}

[public-teleport-github-review-bot]

@tigrato See the table below for backport results.

Branch	Result
branch/v15	Failed
branch/v16	Failed
branch/v17	Failed