Changelog template

[ThomSerg]

As mentioned in #666, now that we'll again be using the changelog.md file for documenting the changes included in each release, it would be nice if we decided on a consistent template.

We've used many names for the same subsections before:

• What's Changed
• What's New
• What's new?
• New:
• ...

Either we create our own template, use the one from keepachangelog or make some hybrid. But as seen in release 0.9.24, sometimes we might want to break out of a strict template as to for example better highlight our efforts on performance improvements. The format included in this pull request is in the "keepachangelog" format.

Any thoughts / opinions?

[ThomSerg]

Maybe the dropping of Python version 3.7 should be under the label Deprecated / Removed (if following the "keepachangelog" convention)

[IgnaceBleukx]

Regarding the classes we want in the changelog, I think it makes sense to mimic the labels we have on our PRs?
I.e., "Documentation", "Bug", "Enhancement"(can be user experience or performance) or "New solver"

[hbierlee]

I like the keepachangelog format and to be consistent. I think even for performance improvements, it's ok to just add them under Changes if I squint at what they mean with each category:

Types of changes

Added: for new features.
Changed: for changes in existing functionality.
Deprecated: for soon-to-be removed features.
Removed: for now removed features.
Fixed: for any bug fixes.
Security: in case of vulnerabilities.