Update CONTRIBUTING.md

[pljones]

What is the current behaviour and why should it be changed?

"the minimum supported Qt version, which is currently 5.9"
This has changed under #3288. The absolute minimum is now 5.12.2.

Describe possible approaches

1. CONTRIBUTING.md must be updated
2. The release notice should also make the dependency change clear
3. I guess the wiki needs updating, too -- so raise a separate PR for that linking to this issue

Has this feature been discussed and generally agreed?

Yep.

[pljones]

3. I guess the wiki needs updating, too -- so raise a separate PR for that linking to this issue

I couldn't find any mention of required dependencies. That makes sense:

• Windows, Mac, iOS, Android - libraries are bundled
• Linux - uses .deb APT dependency management to install libraries

[pljones]

2. The release notice should also make the dependency change clear

From a user perspective this should only affect Linux users on APT systems without available Qt 5.12.2 versions, right? So Ubuntu 18 and older and Debian 10 and older. Ubuntu 20 has 5.12.8 available, which is newer. Debian 11 has 5.15.2 available, which is newer. So I don't think we need to say anything in the release notice, either.