Standardize Airflow Variable names to uppercase

[davcortez]

Fixes

Fixes WordPress/openverse#1507 by @AetherUnbound

Description

• Standardize the following Airflow Variable names to uppercase: slack_message_override, silenced_slack_notifications, environment, ingestion_limit
• Update the references to the lower case Variables mentioned that appear within documentation.

Testing Instructions

Run just test

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
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• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[obulat]

I've searched the repository, and couldn't find any lower-case variable names. Great that the docs are also fixed! Thank you for your contribution, @davcortez !

[AetherUnbound]

This is great, thanks for making this change to both the documentation and the code! Congrats on your first contribution 😄

[davcortez]

Thanks @AetherUnbound @obulat