blog post for release 1.22 (#6632)

* blog post for release 1.22

* fix formatting, drop table of contents

* adjust headings

* lump networking under serving

* put operator first

* fix some minor edits

Author: dprotaso

docs/blog/.nav.yml

@@ -5,6 +5,7 @@ nav:
     - Blog:
       - index.md
       - Releases:
+          - releases/announcing-knative-v1-22-release.md
           - releases/announcing-knative-v1-21-release.md
           - releases/announcing-knative-v1-20-release.md
           - releases/announcing-knative-v1-19-release.md