sys-apps/ignition: pull 2.20.0

[tormath1]

This PR bumps Ignition to 2.20.0 which:

• brings Proxmox support (related to: OEM: Provide Proxmox images #1783)
• Drops two upstreamed patches (akamai: fix base64 decoding coreos/ignition#1893 and sgdisk: Run partx after partition changes coreos/ignition#1717)
• Pulls an Akamai change already tested on Flatcar (sys-apps/ignition: pull akamai patch #2307)
• Promotes v3_5_experimental to a stable spec

Testing done

• Locally built

core@localhost ~ $ sudo cat /var/run/ignition.json
{"ignition":{"config":{"replace":{"verification":{}}},"proxy":{},"security":{"tls":{}},"timeouts":{},"version":"3.6.0-experimental"},"kernelArguments":{},"passwd":{"users":[{"name":"core","sshAuthorizedKeys":["ssh-rsa AAAA..."]}]},"storage":{},"systemd":{}}core@localhost ~ $
core@localhost ~ $ ignition --version
Ignition 2.20.0

---

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.
• CI: http://localhost:8080/job/container/job/packages_all_arches/4881/cldsv/ w/ all platforms