Update check_api_change.sh script to reflect default branch name

[Sam-Burns]

To fix the check_api_change.sh build check, change branch name to reflect current default branch of repo.

Building on the workaround for the check_api_change.sh script failing, this change would make the script run correctly again, and reinstate its use in the runtests.sh build checks.

The underlying problem appeared relate to a change in the default branch name of the repository, from master to main. Corresponding changes in the script seem to be needed.

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