Update dependency flutter_lints to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
flutter_lints (source)	dev_dependencies	major	^4.0.0 → 6.0.0

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Release Notes

flutter/packages (flutter_lints)

v6.0.0

Compare Source

v5.0.0

Compare Source

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Configuration

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♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

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