Fix constraints generation scripts

[potiuk]

We are now generating constraints with better description, and
we include information about DEFAULT_BRANCH (main/v2-1-test etc.)

The scripts to generate the constraints need to get teh variable
passed to docker.

Also names of generated files were wrong. The constraints did
not update the right constraint files.

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^ Add meaningful description above

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[potiuk]

Hey @kaxil @ashb @jhtimmins @uranusjr -> this is the change needed to update the constraint generation after I added the descriptions in constraint files. I already generated new constraints in main, but I need to merge this one and cherry-pick it to v2-1-test to rebuild constraints there.

The constraints have now headers explaining what are they needed for, which branch they were generated for and when, so that it is easier to understand what each constrain set is for:

Example main constraints just generated:

• https://github.com/apache/airflow/blob/constraints-main/constraints-3.6.txt
• https://github.com/apache/airflow/blob/constraints-main/constraints-no-providers-3.6.txt
• https://github.com/apache/airflow/blob/constraints-main/constraints-source-providers-3.6.txt

[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.