platform/packages: Keep around one earlier python version

[osnyx]

To allow easier platform version upgrades, keep around one additional older python version. This is useful e.g. for batou deployments that have not been upgraded yet.
The priorities are already adjusted at nixpkgs side such that python3 still calls the executable provided by the python3 package.

For the 24.11 platform, this adds python311 to the path of all systems.

@flyingcircusio/release-managers

Release process

• Created changelog entry using ./changelog.sh

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• ticket state set to Pull request ready
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.

Security implications

• Security requirements defined? (WHERE)
  • do not introduce any known regression
  • allow easier upgrades by keeping around neighbouring python versions by default
  • prevent stale version over time: don't hardcode a particular old python version
• Security requirements tested? (EVIDENCE)
  • automated tests still pass
  • derive previous python3 version from current python3 default version to prevent version from becoming stale, when forgotten to be updated in new platform releases