move changelog generation to changie

[DanielMSchmidt]

This PR does the groundwork for moving our changelog generation from a manual process to changie.
After this change every PR can have one or multiple changelog entries added to them. We use these to compile the changelog for releases of all kinds. Every changelog entry can currently link to only one issue (if have multiple issues being resolved in one PR you need to add the links manually to your changelog content).

The script in this PR is intended to be used by maintainers when building out a new release and should facilitate most of the currently manual steps within this repo when doing releases.

I split up the experiments / previous releases content to easier insert a new line for the current release when creating everything for the next minor version. sed can be a bit annoying with the differences between osx and linux.

Target Release

1.11.x

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[xiehan]

Is it worth changing the language in these comments now?

[DanielMSchmidt]

Good point @xiehan, we don't need that comment at all anymore :)

[liamcervante]

I think we could update this to include the new changelog process as well? https://github.com/hashicorp/terraform/blob/main/.github/CONTRIBUTING.md#pull-request-lifecycle

We could also maybe add a little documentation in there around exactly what has to go into the changelog entries?

[liamcervante]

should this be in the other file?

[DanielMSchmidt]

Yeah, I mean technically yes, but I could not find a nice way to sed a new line for the last minor version into the list of last versions so I chose to split it up in a way that I can just append to the top :D

[liamcervante]

ah I see, that's what you meant in the description - I understand now 😅

[DanielMSchmidt]

@liamcervante yeah that does make sense, I added a line about the changelog entries 👍