Make sure that weekdays displayed in newsletter scheduler are the same regardless the current user timezone

[caiosba]

Description

Previously, the implementation was using new Date(x, y, z), which takes into account the current user timezone. So, for a user browsing using timezone Asia/Calcutta, for example, the weekdays displayed in the newsletter scheduler started by Monday. For another timezone, for example America/New_York, the weekdays started by Sunday. The fix is to pass the UTC timezone to new Date(). This PR also includes unit tests that reproduce this issue.

Fixes: CV2-4845.

Type of change

• Performance improvement and/or refactoring (non-breaking change that keeps existing functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Security mitigation or enhancement
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Automated test (add or update automated tests)

How has this been tested?

TDD. Unit tests added.

Checklist

• I have performed a self-review of my own code
• I've made sure my branch is runnable and given good testing steps in the PR description
• I considered secure coding practices when writing this code. Any security concerns are noted above.
• I have commented my code in hard-to-understand areas, if any
• I have made needed changes to the README
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• If I implemented any new components, they are self-contained, their propTypes are declared and they use React Hooks and, if data-fetching is required, they use Relay Modern with fragment containers
• If my components involve user interaction - specifically button, text fields, or other inputs - I have added a BEM-like class name to the element that is interacted with
• To the best of my knowledge, any new styles are applied according to the design system
• If I added a new external dependency, I included a rationale for doing so and an estimate of the change in bundle size (e.g., checked in https://bundlephobia.com/)