Move the trigger branch definition into script/utils.sh

What does this MR do and why?

This MR addresses an incorrectly hard-coded CNG branch name used in rails-production-server-boot-puma-cng job, as reported in !136764 (comment 1649020988).

Re: Make sure CNG pipeline uses the branch that mat... (#429717 - closed)

How to test

1. The updated script can be found working in the following 2 jobs where the value of TRIGGER_BRANCH is set to 16-6-stable after stubbing the target branch to be 16-6-stable-ee:

• rails-production-server-boot-puma-cng
• trigger-monibus-env.

2. Without the stubbing from step 1, TRIGGER_BRANCH then holds the value of master because this MR is actually targeting the master branch:

• rails-production-server-boot-puma-cng
• trigger-omnibus-env

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