[TF-M] Start of TF-M v2.1.0 release

Hi,

We are starting the new TF-M release process on the branch release/2.1.xhttps://git.trustedfirmware.org/TF-M/trusted-firmware-m.git/+/refs/heads/release/2.1.x process and freezing the project features in it. RC1 tag will follow shortly.

Let me remind that the code is not frozen, and development can be continued on the main branch as described in TF-M Release Processhttps://tf-m-user-guide.trustedfirmware.org/releases/release_process.html#release-cadence-and-process.

Thanks, Anton