[2021] Janna's ult should be reworked to be not easily cancelled if they're gonna do it for Katarina

We've discussed changes to these abilities before but the main problem is despite all being channels you cannot treat them all the same. Janna in particular is tricky because unlike Kataria her ultimate has multiple, often opposing use cases. In Kat's case any time you cast her ult and cancel is in the first .25 seconds it's a mistake. It is just flatly the wrong decision so we can make an easy decision to mitigate these scenarios.

In Janna's case we originally had it harder to cancel and the feedback was overwhelmingly negative from Janna players becuase Janna's ult can vary from wanting the entire perfect full channel for maximum healing and the absolute minimum amount of healing to just leverage the knockback and move again as quickly as possible. This is a much harder balance to make as we can't predict player intent and the use cases are so extremely different from a controls perspective. I expect the team would be happy to look into adjustments here down the line, but I wanted to say why we made the calls we did and how we got here.