Bug 1878654 - bump documentated supported build targets from 10.12to 10.15 on macOS. r=ahochheiden DONTBUILD

The minimum supported build target got bumped from macOS 10.12 to 10.15 in bug 1840752.

Differential Revision: https://phabricator.services.mozilla.com/D200632

build/docs/supported-configurations.rst

@@ -99,7 +99,7 @@ The **Tier-1 Firefox platforms** and their supported compilers are:
 
 -  Android on Linux x86, x86-64, ARMv7 and ARMv8-A (clang)
 -  Linux/x86 and x86-64 (gcc and clang)
--  macOS 10.12 and later on x86-64 and AArch64 (clang)
+-  macOS 10.15 and later on x86-64 and AArch64 (clang)
 -  Windows/x86, x86-64 and AArch64 (clang-cl)
 
 Prior to Firefox 63, Windows/x86 and Windows/x86-64 relied on the MSVC