feat: use content-type provided by prometheus client, pin airflow 2.6.1 dependencies.

[seruman]

Prometheus >= 3.2.1, seems to reject text content-type by default, unless fallback_scrape_protocol is given for the target. Instead of setting fallback on Prometheus, responding with correct/expected content-type would be better.

CONTENT_TYPE_LATEST = 'text/plain; version=0.0.4; charset=utf-8'

Error scraping target: received unsupported Content-Type "text" and no fallback_scrape_protocol specified for targe

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• Airflow 2.6.1 seems to not have upper bound for most of its dependencies, which some of them had breaking changes. Added an extra called airflow-2.6.1 to pin Airflow itself and add upper bounds to the dependencies causing issues.
• Removed explicit install of wtforms==2.3.3 on CI. Not sure why it was pinned but I think it makes more sense to let Airflow bring whatever version it needs.
• Had to add airflow dags reserialize on CI to make sure DAGs are serialized in to database. Not sure how it was working before. On locally, unless I run scheduler as daemon DAGs are not serialized causing airflow dags unpause <dagname> to fail with; DAG: <dagname> does not exist in 'dag' table. I think it also makes more sense to run reserialize explicitly to prepare environment.