Portage-stable full alignment with Gentoo upstream / GH actions for package updates

[t-lo]

Issue

• Portage-stable repo contains plain copies of Gentoo upstream recipes
• Some packages still significantly outdated
• Some of the build tooling (eclasses, python, etc) still significantly outdated

Impact

• No direct benefit from Gentoo upstream work
• Manual maintenance / copy+paste work necessary to upgrade packages

Ideal future state

• portage-stable is a zero-touch fork of gentoo upstream and we update by forward-winding portage-stable to new gentoo upstream commit IDs
• GH Actions are in place to upgrade packages as soon as upgrade is available in upstream Gentoo (similarly to existing kernel, rust, systemd automation)

Related issues:

• migrate to 17.1 profiles #428

Initial TODO list:

• move out all custom things out from portage-stable into coreos-overlay [RFE] Move out customizations out from portage-stable #646
• update packages in portage-stable to be outdated at most by a revision, not version or suffix - [RFE] Keep packages in portage-stable updated #645
• prepare an initial automation for updating forked packages in coreos-overlay based on our gentoo snapshot [RFE] Update packages in coreos-overlay and add automation to handle them. #648
• come up and implement solution for /etc handling with vanilla packages [RFE] Handle configuration updates in /etc #647
• prepare an initial automation splitting gentoo snapshot update into interesting and not-interesting parts [RFE] Initial automation for updating Gentoo snapshot replacing portage-stable #649