Resolves#7591
This moves our CI to build macOS on Sequoia (macOS 15) with Xcode 26,
including the new macOS 26 beta SDK.
Importantly, this will make our builds on macOS 26 use the new styling.
I've added a new job that ensures we can continue to build with Xcode 16 and
the macOS 15 SDK, as well, although I think that might come to an end
when we switch over to an IconComposer-based icon. I'll verify then. For
now, we continue to support both.
I've also removed our `hasLiquidGlass` check, since this will now always
be true for macOS 26 builds.
This fixes a regression from our Tahoe window styling changes on
earlier, stable versions of macOS. We need to set
"titlebarAppearsTransparent" to true in order to hide the bottom
border.
