Upgrade to heroku-26 stack

[jrochkind]

We are on the one-older heroku-24.

heroku-24 is supported through April 2029, so no rush, but might as well. We are getting warnings on heroku console now on deploy, that a newer stack is available.

[jrochkind]

Good news -- seems to deploy fine with no changes to heroku-26.

Also ./bin/rspec system_env_spec/ passes once deployed on heroku-26.

Seems to Just Work with no need to change anything!

@eddierubeiz Should I upgrade prod to heroku-26? Or too risky before too much vacation, and just wait until late August to do it?

It is not a rush for any reason.

[eddierubeiz]

Oh that's great news. Out of an abundance of caution, I'm going to recommend that we do it once we have a 2-week block of time when we're both not on vacation -- aka September. :)