This is phase 1 of the plan to move all server utils into a single reusable package that can be shared across SSJS SDKs.

Right now it does not do much, just houses the `diagnostics_channel` redis integrations. In subsequent steps:

1. Code will *not* be moved out of `@sentry/core/server` into `@sentry-internal/server-utils`, in v10.
2. Some or all code in `@sentry/core/server` may be moved into `@sentry-internal/server-utils` in v11, as this would be breaking change. (Redis channel integration was not, because it hasn't yet shipped in a release. Other high-level portable components may move over later, if justified.)
3. No components will ever live in *both* places, because `server-utils` depends on `core`, and that would create a type dependency cycle.

`@sentry/core/server` will live on for at least v10, possibly into v11. New server components will be placed in `@sentry-internal/server-utils`.

fix: #21222
fix: JS-2631