refactor: remove dedicated migrations container and run migrations in instrumentation.ts instead

[tefkah]

Issue(s) Resolved

Removes the need for self-hosted users (and us) to keep around and download the gigantic platform-migrations container by instead running migrations in core instead using instrumentation.ts.

High-level Explanation of PR

First, we upgrade to prisma v6. This is not technically necessary, but a good idea to keep up to date i think.

Secondly, add a new instrumentation.migrate.ts file, which just runs the migrations using prisma migrate deploy. It will auto use the correct env vars that next also has access to.

Finally, remove the init container in our terraform config, and remove the migrations container in the self hosting config.
We still build this container because we use it as the bastion. We should probably not upload it to ghcr anymore tho, as it serves no purpose there.

Test Plan

Make sure tests pass, check if i forgot to replace a migration somewhere else!

Screenshots (if applicable)

Notes