[ptl][release] Stepping down as Release Management PTL

Hello all, I'm sorry to say that I have insufficient time and resources to dedicate to being the kind of PTL the community deserves. With that in mind I've asked if Sean has the time to see out the role for the remainder of the Train cycle and he's agreed.

I have proposed https://review.opendev.org/675246 up update the governance repo.

I'm not going any where but instead trying to take on less and do *that* better.

Yours Tony.