Lower macOS deployment target to 10.13

[cbjeukendrup]

Back in the days, we chose 10.14, because:

• We still believed that we would switch to Qt 6 with MuseScore 4.1
• Qt 6 required 10.14
• We wanted to do a OS requirement bump from MuseScore 3.6 to 4.0, not from 4.0 to 4.1

But things turned out differently:

• Switching to Qt 6 is not going to happen anytime soon (there are too many things that are just not working or still missing; and why would we even switch, since the open source world is still receiving LTS updates for Qt 5.15, albeit with one year delay with respect to commercial users)
• Nowadays, Qt 6 requires even 10.15 or even 11.0 (different documentation pages say different things)

So there is no point anymore in artificially limiting MuseScore to macOS 10.14+. This PR lifts that restriction, so that MuseScore 4.2 will run on macOS 10.13 (which is really required, by Qt 5.15).

[cbjeukendrup]

Wow, that's interesting: it only works if we use the latest Xcode version. That's a bit surprising. Apparently they added backdeployment for more C++ std APIs in the latest version.
Anyway, using a newer Xcode version is not a bad thing; and Xcode 15 still supports macOS 10.13 as a deployment target, and there are no unusual warnings printed, so I think it should be fine. Still would be good to actually test this on a (virtual) 10.13 machine.

[cbjeukendrup]

Update about this:

• Zac reported (long ago already) that builds from this PR crash on all macOS versions older than 13.0 (something like that)
• That turns out to be a problem with the linker in Xcode 15.0
• Xcode 15.1 fixes this, and is out since Dec 5th; and now finally also available on GitHub Actions. So let's try again!

[shoogle]

Works for me on macOS 11.6.8. That's the only version I have access to currently.

[bkunda]

With a move to Qt6 on the horizon (which will support macOS 10.14), it is no longer necessary for us to lower the deployment target to 10.13. As such, this PR can be closed.

[cbjeukendrup]

(which will support macOS 10.14)

It doesn't; it doesn't even support 10.15, but only 11 and newer, and soon only 12 and newer. So switching to Qt 6 will cause a huge bump in our system requirements.

Also, lowering the deployment target wasn't necessary, but a welcome feature for some people.

[igorkorsukov]

(which will support macOS 10.14)

It doesn't; it doesn't even support 10.15, but only 11 and newer, and soon only 12 and newer. So switching to Qt 6 will cause a huge bump in our system requirements.

Also, lowering the deployment target wasn't necessary, but a welcome feature for some people.

We can switch to 6.2 first, but it has its own problems, we’ll try to solve them.