replace private _get_credentials by public get_credentials

[zazaho]

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, mypy and type annotations). Our pre-commits will help you with that.
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[potiuk]

Some more context in commit maybe? What's the reason ?

[briva]

Same problem : #28573

[lwyszomi]

@potiuk after this change #25785 we have issue, probably someone by mistake forgot to add change to the Compute Engine hook

[boring-cyborg]

Awesome work, congrats on your first merged pull request!