doc: mention that macOS 10.10 is now required

doc/build-osx.md

@@ -91,6 +91,6 @@ Other commands:
 Notes
 -----
 
-* Tested on OS X 10.8 Mountain Lion through macOS 10.13 High Sierra on 64-bit Intel processors only.
+* Tested on OS X 10.10 Yosemite through macOS 10.13 High Sierra 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)

doc/release-notes.md

@@ -48,11 +48,14 @@ Compatibility
 ==============
 
 Bitcoin Core is extensively tested on multiple operating systems using
-the Linux kernel, macOS 10.8+, and Windows 7 and newer (Windows XP is not supported).
+the Linux kernel, macOS 10.10+, and Windows 7 and newer (Windows XP is not supported).
 
 Bitcoin Core should also work on most other Unix-like systems but is not
 frequently tested on them.
 
+From 0.17.0 onwards macOS <10.10 is no longer supported. 0.17.0 is built using Qt 5.9.x, which doesn't
+support versions of macOS older than 10.10.
+
 Notable changes
 ===============