Update changelog and version after v1.0.6

Author: github-actions[bot]

CHANGELOG.md

@@ -1,5 +1,9 @@
 # CodeQL Action and CodeQL Runner Changelog
 
+## [UNRELEASED]
+
+No user facing changes.
+
 ## 1.0.6 - 19 Jul 2021
 
 - The `init` step of the Action now supports a `source-root` input as a path to the root source-code directory. By default, the path is relative to `$GITHUB_WORKSPACE`. [#607](https://github.com/github/codeql-action/pull/607)