doc/go1.22: pre-announce dropping macOS 10.15 support

For #64207.
For #23011.

Change-Id: I17ff221718aef03e9cda0ae4eb7b79896933553a
Reviewed-on: https://go-review.googlesource.com/c/go/+/549655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

doc/go1.22.html

@@ -895,6 +895,9 @@ <h3 id="darwin">Darwin</h3>
   On 64-bit ARM-based macOS (the <code>darwin/arm64</code> port),
   the Go toolchain already generates PIE by default.
 </p>
+<p><!-- go.dev/issue/64207 -->
+  Go 1.22 is the last release that will run on macOS 10.15 Catalina. Go 1.23 will require macOS 11 Big Sur or later.
+</p>
 
 <h3 id="arm">Arm</h3>
 <p><!-- CL 514907 -->