Adding Variable.update method and improving detection of variable key collisions

[SamWheating]

Re: Discussion in #17889

A few changes in this PR:

• Update Variable.set() method to take a description argument.
• Update Variable.setdefault() method to take a description argument.
• Adding a Variable.update() method which will throw a KeyError if the Variable doesn't exist, and an AttributeError if it doesn't exist in the Database (since a non-metastore Variable can't be modified)
• Improved logging around key collisions between different variable backends.
• Updated documentation to warn users about key collisions between variable backends.

If a user has a duplicated key in the metastore and an extra secrets backend, then updates to the Variable will update the value in the metastore, but reads will read the value in the additional backend.

This is still the case, but I've improved the logging when this happens and updated the documentation to warn users about this behaviour.

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.