Enable the airflow.utils.timezone ban rule

[FrankYang0529]

Why

• Set up process for sharing code between different components. #53149 added a ruff banned-api entry for airflow.utils.timezone but left it commented out behind # Add this once providers are sorted.
• Use common.compat.sdk for timezone imports in providers #70492 and Use common.compat.sdk for the remaining provider timezone imports #71209 cleared the provider call sites, so we can enable the rule.

How

• devel-common keeps its Airflow 2 fallbacks.
• Uncomment the rule and drop the # Add this once providers are sorted note it was waiting on.

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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