darwin: bump minimum supported version to 10.15

We can't realistically claim to support 10.7 or any version that Apple
no longer supports so let's bump the baseline to something more
realistic.

Refs: libuv#482
Refs: libuv#3405

SUPPORTED_PLATFORMS.md

@@ -3,7 +3,7 @@
 |  System | Support type | Supported versions | Notes |
 |---|---|---|---|
 | GNU/Linux | Tier 1 | Linux >= 2.6.32 with glibc >= 2.12 | |
-| macOS | Tier 1 | macOS >= 10.7 | |
+| macOS | Tier 1 | macOS >= 10.15 | |
 | Windows | Tier 1 | >= Windows 8 | VS 2015 and later are supported |
 | FreeBSD | Tier 1 | >= 10 | |
 | AIX | Tier 2 | >= 6 | Maintainers: @libuv/aix |