Update dependency lints to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
lints (source)	dev_dependencies	major	^5.1.1 → ^6.0.0

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Release Notes

dart-lang/core (lints)

v6.1.0

Compare Source

• recommended:
  • added use_null_aware_elements (#​899)
• Run dart format with the new style.

v6.0.0

Compare Source

• core:
  • added strict_top_level_inference (#​836)
• recommended:
  • added unnecessary_underscores (#​856)
• Require Dart 3.8.

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