update merge script to use stable version of rust

[anthony-gomez-fastly]

Change summary

The merge to main build is failing due to new features being added to rust and us using old versions, updating to use stable

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

[rcaril]

Thanks!