Indicate minimum cluster version is now 1.15 🚓

In #1894 we bumped our k8s
dependencies such that a cluster must now be running at least 1.15.

DEVELOPMENT.md

@@ -91,7 +91,7 @@ configuring Kubernetes resources.
 Docker for Desktop using an edge version has been proven to work for both
 developing and running Pipelines. The recommended configuration is:
 
--   Kubernetes version 1.11 or later
+-   Kubernetes version 1.15 or later
 -   4 vCPU nodes (`n1-standard-4`)
 -   Node autoscaling, up to 3 nodes
 -   API scopes for cloud-platform
@@ -113,7 +113,7 @@ kubectl config use-context docker-for-desktop` To setup a cluster with GKE:
     variable (e.g. `PROJECT_ID`).
 
 1.  Create a GKE cluster (with `--cluster-version=latest` but you can use any
-    version 1.11 or later):
+    version 1.15 or later):
 
     ```bash
     export PROJECT_ID=my-gcp-project
@@ -131,7 +131,7 @@ kubectl config use-context docker-for-desktop` To setup a cluster with GKE:
      --machine-type=n1-standard-4 \
      --image-type=cos \
      --num-nodes=1 \
-     --cluster-version=latest
+     --cluster-version=1.15
     ```
 
     Note that

docs/install.md

@@ -9,7 +9,7 @@ Use this page to add the component to an existing Kubernetes cluster.
 
 ## Pre-requisites
 
-1. A Kubernetes cluster version 1.11 or later (_if you don't have an existing
+1. A Kubernetes cluster version 1.15 or later (_if you don't have an existing
    cluster_):
 
    ```bash
@@ -34,7 +34,7 @@ Use this page to add the component to an existing Kubernetes cluster.
 
 The versions of Tekton Pipelines available are:
 
-* [Officially released versions](https://github.com/tektoncd/pipeline/releases), e.g. `v0.6.0`
+* [Officially released versions](https://github.com/tektoncd/pipeline/releases), e.g. `v0.10.0`
 * [Nightly releases](../tekton/README.md#nightly-releases) are
   published every night to `gcr.io/tekton-nightly`
 * `HEAD` - To install the most recent, unreleased code in the repo see