Automate instance preview generation process

[aboydnw]

Context

It takes developer's valuable time to update veda-ui and create previews for testing. We would like to make this easier for us to do as well as for any instance owners/contributors to conduct these releases. This issue is adjacently related to #1236

Acceptance Criteria

• [ ] When a veda-ui release is triggered, PRs for selected instances (GHG, earth.gov, and Earthdata) are created for testing purposes
• [ ] When a veda-ui release is triggered, appropriate team members are tagged on the release PR(s) and/or notified through slack

[hanbyul-here]

@aboydnw I edited the ticket so it is more catered towards creation of preview URLs, not the release of instances.