Add 0.18 forward-porting into 0.19

[kubukoz]

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Added build-plugins integration tests (when reflection loading is required at codegen-time)
• Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• Updated dynamic module to match generated-code behaviour
• Added documentation
• Updated changelog

[Baccata]

besides the fact that this actually sets up fore-port (not back-port), do you want to revisit this in order to automate the process a little more ?

[kubukoz]

yes. PR is called "backport action" because that's how Mergify calls it in the config ;)

I may try revisiting it now that you've sniped me

[kubukoz]

status-success apparently isn't a thing - https://docs.mergify.com/configuration/conditions/

[kubukoz]

@Baccata that should do it. Tested in a repo of mine (without the required status check, but I trust that'll work properly) - kubukoz/demos#8

[kubukoz]

question: do we need a label? I'd say we should do it automatically (that way it'll be much less trouble with things like Steward), or with opt-out via a "do not port" label.