Merge branch 'docs-remove-kubernetes-1.13-add-1.18' into 'master'

Remove support for Kubernetes 1.13, add 1.18

See merge request gitlab-org/gitlab!48503

doc/user/project/clusters/index.md

@@ -52,11 +52,11 @@ version. The range of supported versions is based on the evaluation of:
 GitLab supports the following Kubernetes versions, and you can upgrade your
 Kubernetes version to any supported version at any time:
 
+- 1.18
 - 1.17
 - 1.16
 - 1.15
 - 1.14 (deprecated, support ends on December 22, 2020)
-- 1.13 (deprecated, support ends on November 22, 2020)
 
 Some GitLab features may support versions outside the range provided here.