Roles: Update release team members

[pllim]

In the past several releases (and we only do a release every 6 months, so it's been a while), only @astrofrog and @saimn are alternating, so based on that, I am updating the Release Team membership.

Affected people in this listing:

• @embray
• @bsipocz

And, I have to confess that I accidentally already updated the actual "Astropy Project Release Team" GitHub membership. I thought this was already done over here but I must have imagined it. So, if this PR ends up changing, I'll update on GitHub side accordingly. I take full responsibility for this mistake and I apologize for any inconvenience caused.