docs(prerequisites): update MbedTLS version to 3.6.1

This new update to the LTS branch of MbedTLS provides minor
enhancements and bug fixes; including some security
fixes, and a fix to a compilation warning which
previously affected TF-A.
Full patch notes to this MbedTLS update can be found at
https://github.com/Mbed-TLS/mbedtls/releases/tag/mbedtls-3.6.1.

(Adapted from original commit due to rework of prerequisites.rst file.)

Change-Id: I1a68dfcb52a8361c1689cb6ef12d265a6462fda3
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
(cherry picked from commit 5acc316466bdde9257e8076133d7fc8913f73bf8)
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 9180fb5..ffdc3e2 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -71,7 +71,7 @@
 The following libraries are required for Trusted Board Boot and Measured Boot
 support:
 
-- mbed TLS == 3.6.0 (tag: ``mbedtls-3.6.0``)
+- mbed TLS == 3.6.1 (tag: ``mbedtls-3.6.1``)
 
 These tools are optional: