Update build notes for dropping osx 10.7 support

Github-Pull: #9022
Rebased-From: 1d1246314f761b3f84c869cb7b0af49cc25b55b3

Author: fanquake

doc/build-osx.md

@@ -90,6 +90,6 @@ Uncheck everything except Qt Creator during the installation process.
 Notes
 -----
 
-* Tested on OS X 10.7 through 10.11 on 64-bit Intel processors only.
+* Tested on OS X 10.8 through 10.12 on 64-bit Intel processors only.
 
 * Building with downloaded Qt binaries is not officially supported. See the notes in [#7714](https://github.com/bitcoin/bitcoin/issues/7714)