doc: bump minimum requirement to OS X 10.7

Fixes golang#11995

Change-Id: I9e2901d77ebde705f59822e7d4a8163cbacffcd7
Reviewed-on: https://go-review.googlesource.com/13150
Reviewed-by: Rob Pike <r@golang.org>

doc/install-source.html

@@ -399,7 +399,7 @@ <h2 id="environment">Optional environment variables</h2>
 
 <p>
 Choices for <code>$GOOS</code> are
-<code>darwin</code> (Mac OS X 10.6 and above and iOS), <code>dragonfly</code>, <code>freebsd</code>,
+<code>darwin</code> (Mac OS X 10.7 and above and iOS), <code>dragonfly</code>, <code>freebsd</code>,
 <code>linux</code>, <code>netbsd</code>, <code>openbsd</code>,
 <code>plan9</code>, <code>solaris</code> and <code>windows</code>.
 Choices for <code>$GOARCH</code> are

doc/install.html

@@ -15,7 +15,7 @@ <h2 id="download">Download the Go distribution</h2>
 <p>
 <a href="https://golang.org/dl/" target="_blank">Official binary
 distributions</a> are available for the FreeBSD (release 8-STABLE and above),
-Linux, Mac OS X (Snow Leopard and above), and Windows operating systems and
+Linux, Mac OS X (10.7 and above), and Windows operating systems and
 the 32-bit (<code>386</code>) and 64-bit (<code>amd64</code>) x86 processor
 architectures.
 </p>
@@ -47,7 +47,7 @@ <h2 id="requirements">System requirements</h2>
 <tr><td colspan="3"><hr></td></tr>
 <tr><td>FreeBSD 8-STABLE or later</td> <td>amd64, 386, arm</td> <td>Debian GNU/kFreeBSD not supported; FreeBSD/ARM needs FreeBSD 10 or later</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; no binary distribution for ARM yet</td></tr>
-<tr><td>Mac OS X 10.6 or later</td> <td>amd64, 386</td> <td>use the gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></td></tr>
+<tr><td>Mac OS X 10.7 or later</td> <td>amd64</td> <td>use the gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></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>