build: set minimum supported macOS to 10.12

doc/build-osx.md

@@ -110,7 +110,7 @@ tail -f $HOME/Library/Application\ Support/Bitcoin/debug.log
 ```
 
 ## Notes
-* Tested on OS X 10.10 Yosemite through macOS 10.14 Mojave on 64-bit Intel
+* Tested on OS X 10.12 Sierra through macOS 10.15 Catalina 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).