Archive silk repo, it was merged into silk-release

We had originally setup silk in an enitre other repo with the intention
that it could be imported, used, and extended by other projects.
This never happened and now we have a separate repo for `silk` when it
is only used in `silk-release.

We have moved `silk` into `silk-release` and therefore `silk` is no
longer necessary.

More details:
- cloudfoundry/silk#52
- cloudfoundry/silk-release#89

[#185149418](https://www.pivotaltracker.com/story/show/185149418)

org/cloudfoundry.yml

@@ -2141,6 +2141,7 @@ orgs:
         has_projects: true
         private: true
       silk:
+        archived: true
         allow_merge_commit: false
         default_branch: main
         description: a network fabric for containers.  inspired by flannel, designed

toc/working-groups/app-runtime-platform.md

@@ -369,7 +369,6 @@ areas:
   - cloudfoundry/routing-perf-release
   - cloudfoundry/routing-release
   - cloudfoundry/routing-team-checklists
-  - cloudfoundry/silk
   - cloudfoundry/silk-release
 
 - name: Networking-Extensions