[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_2764.prdoc

@@ -0,0 +1,16 @@
+title: Validator disabling in Backing.
+
+doc:
+  - audience: Node Operator
+    description: |
+      Once a validator has been disabled for misbehavior, it will no longer
+      sign backing statements in the current era.
+
+migrations:
+  db: []
+  runtime: []
+
+crates:
+  - name: polkadot-node-core-backing
+
+host_functions: []