doc: document minimum OS X version as 10.8

The download page says "OS X 10.8 or later", but other pages said 10.7.
Say 10.8 everywhere.

Turns out Go doesn't even compile on OS X 10.7 (details in bug) and we
only run builders for OS X 10.8+, which is likely why 10.7
regressed. Until recently we only had OS X 10.10 builders, even.

We could run 10.7 builders, but there's basically no reason to do so,
especially with 10.12 coming out imminently.

Fixes golang#16625

Change-Id: Ida6e20fb6c54aea0a3757235b708ac1c053b8c04
Reviewed-on: https://go-review.googlesource.com/28870
Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/install-source.html

@@ -446,7 +446,7 @@ <h2 id="environment">Optional environment variables</h2>
 
 <p>
 Choices for <code>$GOOS</code> are
-<code>darwin</code> (Mac OS X 10.7 and above and iOS), <code>dragonfly</code>, <code>freebsd</code>,
+<code>darwin</code> (Mac OS X 10.8 and above and iOS), <code>dragonfly</code>, <code>freebsd</code>,
 <code>linux</code>, <code>netbsd</code>, <code>openbsd</code>,
 <code>plan9</code>, <code>solaris</code> and <code>windows</code>.
 Choices for <code>$GOARCH</code> are

doc/install.html

@@ -49,7 +49,7 @@ <h2 id="requirements">System requirements</h2>
 <tr><td colspan="3"><hr></td></tr>
 <tr><td>FreeBSD 8-STABLE or later</td> <td>amd64</td> <td>Debian GNU/kFreeBSD not supported</td></tr>
 <tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm</td> <td>CentOS/RHEL 5.x not supported</td></tr>
-<tr><td>Mac OS X 10.7 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup> for <code>cgo</code> support</td></tr>
+<tr><td>Mac OS X 10.8 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup> for <code>cgo</code> support</td></tr>
 <tr><td>Windows XP or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cygwin or msys.</td></tr>
 </table>