Safa/restore discounted amount deprecated cop

[safafa]

Description

This PR restores solidus/discounted_amount as a warning cop
issue 31

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Severity:

• - info
• - refactor
• - convention (default)
• - warning
• - error
• - fatal

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Wrong Code

line_item.discounted_amount

Correct Code

line_item.total_before_tax

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solidusio/solidus@5d61419

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Before submitting the PR make sure the following are checked:

• The PR relates to only one cop with a clear title and description.
• Feature branch is up-to-date with main (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran and ensured all tests are passing on a development environment.
• If this is a new cop, added an entry for the cop on /config/default.yml
• Updated Changelog