Bump minimum macOS SDK to 10.15

According to https://firefox-source-docs.mozilla.org/widget/cocoa/sdks.html Firefox doesn't support anything lower than 10.15 at runtime. So neither do we.
Fixes #7370

build/jenkins/pipelines/bundles.Jenkinsfile

@@ -36,7 +36,7 @@ pipeline {
 	}
 
 	environment {
-		MIN_OSX_VERSION = "10.12"
+		MIN_OSX_VERSION = "10.15"
 	}
 
 	stages {

build/workspaces/update-workspaces.sh

@@ -31,7 +31,7 @@ done
 
 if [ "$OS" = "Darwin" ]; then
 	# Set minimal SDK version
-	: "${MIN_OSX_VERSION:=10.12}"
+	: "${MIN_OSX_VERSION:=10.15}"
 	: "${WX_CONFIG:=$(realpath ../../libraries/macos/wxwidgets/bin/wx-config)}"
 	export MIN_OSX_VERSION WX_CONFIG
 fi

libraries/build-macos-libs.sh

@@ -62,7 +62,7 @@ MOLTENVK_VERSION="1.2.2"
 # --------------------------------------------------------------
 
 export CC="${CC:="clang"}" CXX="${CXX:="clang++"}"
-export MIN_OSX_VERSION="${MIN_OSX_VERSION:="10.12"}"
+export MIN_OSX_VERSION="${MIN_OSX_VERSION:="10.15"}"
 export ARCH="${ARCH:=""}"
 
 # The various libs offer inconsistent configure options, some allow