Releases: Adjust markdown format

When services like https://github.com/renovatebot/renovate open dependency update pull requests they try to find a corresponding changelog entry to let the developers know what changes this dependency update includes.

The current format of the `RELEASES.md` file with its nested lists appears to be uncommon enough to not get recognized by at least renovatebot.

This changes the format to use `h2` elements for the versions, which should allow renovatebot to pick up the contents and generally renders in a slightly nicer way :D