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.

Change-Id: I1a68dfcb52a8361c1689cb6ef12d265a6462fda3
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
1 file changed