Clean up codes for QEMU < 7.0

[AkihiroSuda]

• QEMU 7.0 is documented as the minimum requirement since 9eb1396 (Dec 2022)
• Also clean up the hint for QEMU 8.2.0, as QEMU 8.2.0 users should have already updated QEMU to 8.2.1+

[jandubois]

I agree with bumping up the minimum version of QEMU, but if we remove warnings for older macOS versions, then we should have a top-level check that explains that Lima is no longer supported on that OS version instead of letting users run into problems.

However, I would disagree with breaking Lima on macOS 12, as it is often the latest version older Intel Macs can be upgraded to (e.g. I personally don't have any local Intel Mac that can run macOS 13 or later, but these are quad-core 4 GHz i7 machines with 16-64 GB of RAM and work otherwise totally fine using macOS 12 Monterey).

Yes, Apple considers this hardware (which is barely 10 years old) obsolete, and Homebrew doesn't support it anymore either. So I don't think we should spend any effort actively supporting it. But we shouldn't break it either unless it gets in the way of implementing new functionality.

[AkihiroSuda]

Restored the codes for macOS 12

[afbjorklund]

I think originally the request was to be able to use Lima with the system version of QEMU,
especially since one use case was to run the newer kernel in an Lima instance if needed...

Ubuntu	QEMU
20.04	4.2
22.04	6.2
24.04	8.2

RHEL	QEMU
7	1.5
8	6.2
9	9.0

So it would be nice to keep the minimum version at 6.2, unless 7.0 is somehow required?
Otherwise one has to build a newer version locally, which makes Lima harder to install.

• docs: find out the oldest QEMU version that is compatible with Lima #320

But since we have already bumped it for Mac, can bump the minimum version for Linux too.

Then the users can choose, if they are able to upgrade their OS or if they want to install QEMU.

[AkihiroSuda]

Maybe we should allow QEMU 6.2 only on linux/amd64 host?
We should also have a ubuntu-22.04 CI?