[TF-M] Start of TF-M v1.6.0 release process

Hello,

The branch release/1.6.xhttps://git.trustedfirmware.org/TF-M%2Ftrusted-firmware-m.git/log/?h=refs%2Fheads%2Frelease%2F1.6.x has been created, indicating the project feature's freeze and beginning the release process. Expecting to place RC1 tag asap, after successful run of the basic tests.

Let me remind that the code is not frozen, and development can be continued on the main branch.

Thanks, Anton