Use https://keepachangelog.com format

I think this is the line that replaces the ## Unreleased header to ## <version-number for a release: https://github.com/getsentry/craft/blob/2e4f74c618ce99c184d4a4643fd92abda5ad0724/src/commands/prepare.ts#L399

Two proposals to improve our workflow here:

1. Instead of replacing ## Unreleased, simply insert \n\n## <version-number> beneath it, so the unreleased section is there waiting for when we want to add more things during development, instead of having to rewrite that after every release.
2. To adhere more closely to the https://keepachangelog.com format (because we are about 95% of the way there), we should include the date of the release and enclose the formats in square brackets, as such:

[armcknight]

Understood @BYK. I was only pointing out the need for my proposed change if, for whatever reason, people don’t want to use the automated changelog feature.

[BYK]

@armcknight It does much more than that if you are willing to use it. Here’s a good example: https://github.com/getsentry/craft/releases/tag/0.25.0 – It uses GitHub milestones to group PRs and use a better wording and sectioning for a feature. Some more docs here: https://github.com/getsentry/craft#changelog-policies