Update README for [beta] (#6270)

* Update beta readme to reflect required rust version

* Update Changelog for 1.6.10 and 1.7.0 (#6183)

* Update changelog for 1.6.10

* Update changelog for 1.7.0

* Move 1.7.0 to top

* Rephrase contract warp

* Update final 1.7.0 changelog

* Update beta backports in Changelog

* Update SECURITY.md