Update Mac Legacy from Qt 5.9.9 to 5.15.2

[softins]

Short description of changes

Updates the CI build process and makefiles for Mac Legacy to use Qt 5.15.2 instead of 5.9.9. This allows other things also to be improved separately, as described in #3234

CHANGELOG: Build: Update Mac Legacy build to use Qt 5.15.2, now compatible with MacOS 10.13 (High Sierra) or newer.

Context: Fixes an issue?

Fixes #3234

Does this change need documentation? What needs to be documented and how?

We would need to document in the release notes and any download page that the minimum supported version of MacOS for the Legacy build would be 10.13 (High Sierra)

Status of this Pull Request

Working, but awaiting agreement on using Qt 5.15.2.

What is missing until this pull request can be merged?

Just agreement on using Qt 5.15.2

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[softins]

Reverted to using Qt 5.15.2, as per #3234 (comment)

[softins]

I would like to progress this now, because I plan to submit a working PR to replace #2393, and that will be dependent on this one having been merged.

[ann0see]

You know that I have mixed feelings about this but you can argue that people on those devices should have upgraded years ago.
Going to approve it. But yes, this needs to be explained in the release notes.

[ann0see]

We no longer officially link legacy directly on the website anyway as we didn't test it for the last release. So people know it's encouraged not to use it.

[pljones]

Approving because it builds clean. I've no way to test it.

[softins]

Great, I'll do a final smoke test of the artifact and then merge.