whats-new: Generalize PAC/BTI entry to also cover Intel CET

Turn this entry into a more general hardening entry describing the features for both amd64 and arm64, which got added at the same time, as part of the same hardening effort. And are intended to protect against the same kinds of attacks.

Closes: #1107956

@ema @zeha ok how about this as a starting point? (Happy to reword or rearrange, etc.)