Document server-side dry-run is GA in 1.18 (#19549)

* Update dry-run docs.

* Promote server-side dry-run feature state to stable in 1.18
* Replace --dry-run with --dry-run=server|client|none accordingly
* Update server-side dry-run blog post

* Update content/en/docs/reference/using-api/api-concepts.md

Co-Authored-By: Tim Bannister <tim@scalefactory.com>

* Address comments

* Add note at end of server-side dry-run blogpost

Co-authored-by: Tim Bannister <tim@scalefactory.com>

content/en/blog/_posts/2019-01-14-apiserver-dry-run-and-kubectl-diff.md

@@ -97,3 +97,11 @@ semantics to fields! It's also going to improve support for CRDs and unions!
 - Some kubectl apply features are missing from diff and could be useful, like the ability
 to filter by label, or to display pruned resources.
 - Eventually, kubectl diff will use server-side apply!
+
+{{< note >}}
+
+The flag `kubectl apply --server-dry-run` is deprecated in v1.18.
+Use the flag `--dry-run=server` for using server-side dry-run in
+`kubectl apply` and other subcommands.
+
+{{< /note >}}