`StrictUnusedVariable`: update severity to a warning, and remove suggested fixes to prefix variables with underscores

[mpritham]

Before this PR

The StrictUnusedVariable check currently produces errors when a variable is unused unless it is prefixed with an underscore. There are cases (albeit rare) where users legitimately need to keep a reference to an object in scope to prevent the GC from cleaning it. For these scenarios, the underscore prefix (or a suppression) is introduced to satisfy this check.

By moving the severity to the WARNING level, users will still be alerted of their unused variables, and can decide what action they'd like to take.

After this PR

==COMMIT_MSG== Closes #2361. Change the severity to WARNING. Remove suggested fixes to prefix unused variables with underscores, but keep the suggested fix which renames used variables prefixed with underscores. ==COMMIT_MSG==

Possible downsides?

[changelog-app[bot]]

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Type

• [ ] Feature
• [ ] Improvement
• [ ] Fix
• [ ] Break
• [ ] Deprecation
• [ ] Manual task
• [ ] Migration

Description Closes #2361. Change the severity to WARNING. Remove suggested fixes to prefix unused variables with underscores, but keep the suggested fix which renames used variables prefixed with underscores.

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