[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_2684.prdoc

@@ -0,0 +1,14 @@
+title: Add XCM FungibleAdapter
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      A new AssetTransactor has been added to xcm-builder: FungibleAdapter.
+      It's meant to be used instead of the old CurrencyAdapter for configuring the XCM executor
+      to handle only one asset.
+
+crates:
+  - name: "xcm-builder"
+
+migrations: []
+host_functions: []