Do rolling updates to the Open Saas template

[sodic]

Open Saas suffers from the same migration problems described in #3206 with an extra complication added on top.

In short:

1. Open Saas has its own CI which downloads Wasp using curl.
2. We can't do rolling updates to Open Saas's main branch without braking the CI (because the main version of Wasp is not available through curl).
3. We end up either postponing the migration until the release or building up merge-before-release PRs we often forget about in the Open Saas repo.

End goals:

• We can migrate Open Saas the same way we migrate all example apps.
• Open Saas (as a tempalte) is tested against the main version of Wasp, just like all other starter templates.
• At any point in time, we should be able to do cabal run wasp-cli new -t saas and get a functioning app.

[sodic]

Discussion happening at: #3073
Also blocked by #3073