doc: use past releases in Release Policy explanation

As suggested by Ian on the issue, just use past releases to avoid any
confusion regarding current and future releases.

Fixes #23891

Change-Id: Ie513cd3e15aa04822898be57f71976e6fe6bd816
Reviewed-on: https://go-review.googlesource.com/107078Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/devel/release.html

@@ -16,11 +16,11 @@ git checkout <i>release-branch</i>
 
 <p>
 Each major Go release is supported until there are two newer major releases.
-For example, Go 1.8 is supported until Go 1.10 is released,
-and Go 1.9 is supported until Go 1.11 is released.
+For example, Go 1.5 was supported until the Go 1.7 release, and Go 1.6 was
+supported until the Go 1.8 release.
 We fix critical problems, including <a href="/security">critical security problems</a>,
 in supported releases as needed by issuing minor revisions
-(for example, Go 1.9.1, Go 1.9.2, and so on).
+(for example, Go 1.6.1, Go 1.6.2, and so on).
 </p>
 
 <h2 id="go1.10">go1.10 (released 2018/02/16)</h2>