4.23.0.0 release (#679)

* 4.23.0.0 release

* about.rst: add missing 4.23.0.0 highlights from cwiki feature list

Adds API Key Pair Restructure, KMS+HSM Integration, KBOSS (KVM backup
on secondary storage), Veeam KVM backup integration, scheduled min/max
VM autoscaling, per https://cwiki.apache.org/confluence/spaces/CLOUDSTACK/pages/404160964/4.23.0.0

* remove source/upgrading/upgrade/upgrade-4.23.rst

* revert source/_static/theme_overrides.css

* Update source/releasenotes/about.rst

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Co-authored-by: Fabricio Duarte <fabricio.duarte.jr@gmail.com>