common: Remove PodDisruptionBudget resources for Istio

Istio 1.6.0 generated manifests include some policy/v1
PodDisruptionBudget resources that we need to remove. See:
- istio/istio#12602
- istio/istio#24000

The current manifests utilize two "delete" patches:
- common/istio-1-16/istio-install/base/patches/remove-pdb.yaml
- common/istio-1-16/cluster-local-gateway/base/patches/remove-pdb.yaml

However these patches do not work with kustomize v3.2.0. The root
cause is that v3.2.0 doesn't have the appropriate openapi schema for
the policy/v1 API version resources. This is fixed in kustomize v4+.
See:
- kubernetes-sigs/kustomize#3694 (comment)
- kubernetes-sigs/kustomize#4495

Changes:
- We disable the delete patches until we upgrade to kustomize v4+.
  - tracked in: kubeflow#1797

- As a temporary workaraound we remove PodDisruptionBudget resources
  manually with yq before deploying Istio manifests.

- Update README file with instructions.

Refs: kubeflow#2325

Signed-off-by: Apostolos Gerakaris <apoger@arrikto.com>

common/istio-1-16/README.md

@@ -63,6 +63,33 @@ old version is `X1.Y1.Z1`:
 
     ---
 
+5. Remove PodDisruptionBudget from `istio-install` and `cluster-local-gateway` kustomizations. 
+   See:
+    - https://github.com/istio/istio/issues/12602 
+    - https://github.com/istio/istio/issues/24000
+
+   Until now we have used two patches:
+   - `common/istio-1-16/istio-install/base/patches/remove-pdb.yaml`
+   - `common/istio-1-16/cluster-local-gateway/base/patches/remove-pdb.yaml`
+
+   The above patches do not work with kustomize v3.2.0 as it doesn't have the appropriate 
+   openapi schemas for the policy/v1 API version resources. This is fixed in kustomize v4+. 
+   See:
+   - https://github.com/kubernetes-sigs/kustomize/issues/3694#issuecomment-799700607
+   - https://github.com/kubernetes-sigs/kustomize/issues/4495
+
+   A temporary workaround is to delete these resources manually with `yq`: 
+
+        $ yq eval -i 'select((.kind == "PodDisruptionBudget" and .metadata.name == "cluster-local-gateway") | not)' common/istio-1-16/cluster-local-gateway/base/cluster-local-gateway.yaml
+        $ yq eval -i 'select((.kind == "PodDisruptionBudget" and .metadata.name == "istio-ingressgateway") | not)' common/istio-1-16/istio-install/base/install.yaml
+        $ yq eval -i 'select((.kind == "PodDisruptionBudget" and .metadata.name == "istiod") | not)' common/istio-1-16/istio-install/base/install.yaml
+
+   ---
+   **NOTE**
+
+   NOTE: Make sure to remove a redundant {} at the end of the `common/istio-1-16/istio-install/base/install.yaml` and `common/istio-1-16/cluster-local-gateway/base/cluster-local-gateway.yaml` files.
+   ---
+
 ## Changes to Istio's upstream manifests
 
 ### Changes to the upstream IstioOperator profile