GONRG-5757: Update ref from master to default repo branch
Replaced hard-coded "master" with $CI_DEFAULT_BRANCH variable in release_image_tag.sh.
Newer projects are using 'main' for their default branch, and hard-coding to "master" limits the use of these CI files in such projects. By using $CI_DEFAULT_BRANCH, the logic should work fine in any project.
Edited by Volodymyr Pienskoi [EPAM / GCP]