fix(nix): Update nixpkgs and bump nodejs from 18 to 20

[NickLarsenNZ]

The latest release of stackablectl didn't compile for nix users because the nixpkgs used a version of rust that was too (old 1.82.0).

...checking manifest_path ./crates/stackable-webhook/Cargo.toml
error: failed to parse manifest at `/build/operator-rs-5fdc47a/crates/stackable-webhook/Cargo.toml`

Caused by:
  error inheriting `edition` from workspace root manifest's `workspace.package.edition`

Caused by:
  failed to parse manifest at `/build/operator-rs-5fdc47a/Cargo.toml`

Caused by:
  feature `edition2024` is required

  The package requires the Cargo feature called `edition2024`, but that feature is not stabilized in this version of Cargo (1.82.0 (8f40fc59f 2024-08-21)).
  Consider adding `cargo-features = ["edition2024"]` to the top of Cargo.toml (above the [package] table) to tell Cargo you are opting in to use this unstable feature.
  See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2024 for more information about the status of this feature.

This PR updates nixpkgs, then fixes the remaining issue of nodejs 18 being removed

 error: Node.js 18.x has reached End-Of-Life and has been removed

Tip

In future, we should update nixpkgs before the release (niv update nixpkgs), and
check compilation using:

nix build --extra-experimental-features "nix-command flakes" -f .

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] Helm chart can be installed and deployed operator works
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added