[macOS] Bump min. version to 10.15, and remove deprecated code.

[bruvzg]

Fixes godotengine/build-containers#124

Alternative to #76394.

[akien-mga]

I don't know how trustworthy those market usage stats are (they seem to have a problem tracking macOS versions since the bump to 11): https://gs.statcounter.com/macos-version-market-share/desktop/worldwide/#monthly-202203-202303-bar

But they show that dropping support for macOS 10.13 and 10.14 would cut off 8% of the market, which might be a bit too big if we don't have a strong technical requirement to drop support for them.

Valve has just dropped support for macOS 10.11 and 10.12, so I think it makes sense for us to also keep 10.13 as our minimal supported version: https://help.steampowered.com/en/faqs/view/0972-74BD-D8DE-662E

I tried using the Steam Hardware Survey to get a feel for what percentage of users still use < 10.15, but it has too much granularity in the 10.x.y patch releases so the useful data is all lumped together in a huge 40% "Other" category. https://store.steampowered.com/hwsurvey/Steam-Hardware-Software-Survey-Welcome-to-Steam?platform=mac

[bruvzg]

Some people might keep 10.13 or 10.14 since 10.15+ dropped support for 32-bit software (including 32-bit Windows apps via Wine).

[akien-mga]

That, plus I think a lot of people have older Macs they still want to use, but for which Apple doesn't provide an upgrade path to 10.15, right? So they're effectively stuck on EOL versions, or need to throw out the hardware.

[bruvzg]

Apple doesn't provide an upgrade path to 10.15, right?

I think they only drop support for 2010 Mac Pro in 10.15, the rest of hardware requirements should be the same as 10.14. But in case 10.13 to 10.14, yes, a lot of old (2009-2010) macs were dropper.

[Calinou]

I would prefer keeping 10.13 as a baseline for now too.