Can Heron's ability be used to pay for Yellow Kennen's stun ability ?

No — Heron's discount only reduces the cost of a card you play, and Kennen's stun is a triggered ability cost, not a card play.

Astral Heron's ability (per its errata) reads "the next card you play this turn costs [2][A][A] less." Kennen, Keeper of Balance's stun is a cost to finalize his trigger on the Chain ("you may pay [2] to Stun a unit," Rule 383.3.b, 404.2) — it's an ability, not a card being played. Discounts on ability costs aren't applied unless the effect says so (Rule 206.1), and Heron only speaks to cards. So you still pay the full [2] for the stun, even if Heron's discount is waiting for your next card.