[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
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