Add "NON_PREEMPTIBLE" as a valid value for Dataproc workers

[tskir]

This a very trivial and static change.

Dataproc API allows this value, alongside the other three: https://cloud.google.com/dataproc/docs/reference/rpc/google.cloud.dataproc.v1#google.cloud.dataproc.v1.InstanceGroupConfig.Preemptibility

It is sometimes useful to force non-preemptible secondary workers, for example for benchmarking purposes.

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[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)
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[hussein-awala]

LGTM

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[hussein-awala]

Congrats on your first commit 🎉