docs(security): update info on use of OpenSSL 3.0

OpenSSL 3.0 is a pre-requisite since v2.7 and can be installed
on the operating system by updating the previous version.
However, this may not be convenient for everyone, as some may
want to keep their previous versions of OpenSSL.

This update on the docs shows that there is an alternative to
install OpenSSL on the system by using a local build of
OpenSSL 3.0 and pointing both the build and run commands to
that build.

Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>
Change-Id: Ib9ad9ee5c333f7b04e2747ae02433aa66e6397f3
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 0b8a71c..5d575d8 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -57,6 +57,12 @@
 
    Required to build the cert_create tool.
 
+   .. note::
+
+    OpenSSL 3.0 has to be built from source code, as it's not available in
+    the default package repositories in recent Ubuntu versions. Please refer
+    to the OpenSSL project documentation for more information.
+
 The following libraries are required for Trusted Board Boot and Measured Boot
 support:
 
@@ -89,7 +95,7 @@
 
 .. code:: shell
 
-    sudo apt install build-essential git libssl-dev
+    sudo apt install build-essential git
 
 The optional packages can be installed using: