doc: Change LXR references to elixir.bootlin.com

Recently, Free Electrons was renamed to Bootlin[1]. Less recently, the
Linux Cross Reference (LXR) at lxr.free-electrons.com was replaced by
Elixir[2], and lxr.free-electrons.com redirected first to
elixir.free-electrons.com and now to elixir.bootlin.com.

[1]: https://bootlin.com/blog/free-electrons-becomes-bootlin/
[2]: https://github.com/free-electrons/elixir

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>
Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/howto.rst

@@ -225,7 +225,7 @@ Cross-Reference project, which is able to present source code in a
 self-referential, indexed webpage format. An excellent up-to-date
 repository of the kernel code may be found at:
 
-	http://lxr.free-electrons.com/
+	https://elixir.bootlin.com/
 
 
 The development process

Documentation/process/kernel-docs.rst

@@ -565,7 +565,7 @@ Miscellaneous
 
     * Name: **Cross-Referencing Linux**
 
-      :URL: http://lxr.free-electrons.com/
+      :URL: https://elixir.bootlin.com/
       :Keywords: Browsing source code.
       :Description: Another web-based Linux kernel source code browser.
         Lots of cross references to variables and functions. You can see