configure Nix to use default node version 20.15.1

[stepchud]

Description

Had an issue running the app locally. It was fixed by updating the Nix node version to use the default version (currently 20.15.1).

Related Issues

Changes Made

• shell.nix (only affects dev machines that are using Nix)

Checklist

• [x] PR is assigned to a project: Challenge_gov Board
• [x] PR is assigned to a milestone: current sprint
• [ ] GH issues are assigned to the PR (under Development section on the right hand side)
• [ ] PR is assigned a reviewer, and requires code review and approval by a teammate
• [ ] New functionality is tested and all tests are green
• [ ] I have added unit tests for new functionality or updated existing tests for changes.
• [ ] I have updated the documentation/README accordingly, if necessary.
• [ ] If you have DB migration, it is a "safe" migration and you've confirmed it can be rolled back.
• [ ] If applicable, Controllers modified contain appropriate authorization plugs
• [ ] This PR has been reviewed by at least one other team member.