Build: Update macOS runner (legacy) (macos-10.15)

[ann0see]

Describe the bug

GitHub is deprecating the use of macOS Catalina: actions/runner-images#5583

We'll need to update soon to still support the legacy build.

To Reproduce

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Expected behavior

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Screenshots

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Operating system
macOS

[hoffie]

We currently use Xcode 12.1.1 for the legacy build. This version is no longer available on macos-11 images.
We either need to downgrade to 11.7 or upgrade to 12.4.

Based on the comments, we can't upgrade to 12.4 as it contains SDK 11.x which is not supported by Qt5:

jamulus/.github/workflows/autobuild.yml

Lines 157 to 161 in 340ea33

	# For Qt5 on Mac, we need to ensure SDK 10.15 is used, and not SDK 11.x.
	# Xcode 12.1 is the most-recent release which still ships SDK 10.15:
	# https://developer.apple.com/support/xcode/
	# Xcode 12.1.1 is the most-recent 12.1.x release:
	# https://xcodereleases.com/

Therefore, it seems like the only valid options would be downgrading to 11.7 even if it sounds totally wrong.

[hoffie]

Started a build here:
https://github.com/hoffie/jamulus/runs/7814643184?check_suite_focus=true

https://github.com/hoffie/jamulus/pull/new/autobuild-macos-11-legacy

[ann0see]

The question is if not supported = produces broken build or just untested.

[hoffie]

This issue is almost certainly going to affect us this year. We should somehow figure out how to continue with legacy builds.

1. Do we now which macOS versions require the legacy version at all?
2. Are those macOS versions still supported?
3. Do we know of any users who use such software?

I'd propose to ask at least the latter question in the Discussion section (maybe ping some known Mac users). If they are still on those old releases and have good reasons, we might keep it. Otherwise I'd argue for dropping support for that build.

@ann0see Could you do that?
Pinging @softins here as well in case he has opinions on that... :)

[ann0see]

@hoffie macOS legacy is being used: https://somsubhra.github.io/github-release-stats/?username=jamulussoftware&repository=jamulus&page=1&per_page=5

Ofc. Not that many people in comparison to the normal Mac version, but still a considerable amount.
I do know quite some (not so tech savvy) musicians who use outdated macs/PCs for music applications like my second choir leader.

Especially buying a new Mac is expensive and a lot of music applications work well on these old machines.

[ann0see]

We could of course make a policy shift: provide legacy base version, but if we introduce features which don't work out of the box on old Qt versions, we don't provide workarounds (as for softins' language PR, where we'd drop legacy translation support).

[ann0see]

I think 3.9.1 will be the last version we can build on the old runner.