[Backport] Version bumps from 1.6.0 release (#2965)

This PR backports version bumps from release branch
`release-polkadot-v1.6.0` back to `master` and also moved `prdoc` files
related to the release to the appropriate folder

prdoc/1.6.0/pr_1191.prdoc

@@ -0,0 +1,21 @@
+title: Approve multiple candidates with a single signature
+
+doc:
+  - audience: Node Operator
+    description: |
+      Changed approval-voting, approval-distribution to approve multiple candidate with a single message, it adds:
+      * A new parachains_db version.
+      * A new validation protocol to support the new message types.
+      The new logic will be disabled and will be enabled at a later date after all validators have upgraded.
+
+migrations:
+  db:
+    - name: Parachains database change from v4 to v5.
+      description: |
+        Approval-voting column format has been updated with several new fields. All existing data will be automatically
+        be migrated to the new values.
+
+crates:
+  - name: "polkadot"
+
+host_functions: []