Update roadmap to better reflect feature-based releases

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Checklist

• This issue is not a duplicate, I've searched the Issue Tracker
• This issue is not a question, I'm aware that I can submit questions via Mastodon, Twitter, Email, Reddit or Matrix
• This issue is a content request, and not a bug report, feature request, or anything other

What are you missing or what could be improved?

The current roadmap suggests that the project uses time-based releases, rather than feature-based releases. Also, it somehow mixes up the roadmap and the changelog.

What is the solution you'd like to see?

1. Remove the idea of time-based, quarterly releases
2. Clearly split the roadmap from the changelog part

Do you have additional comments?

N/A