[flutter_lints] Bump lints dependency to 6.0.0 and bump package to 6.0.0

[BenjaminPrice]

Bump the dependency on lints from ^5.0.0 to ^6.0.0 in the flutter_lints package. Subsequently also bumping the version of flutter_lints to 6.0.0 to match.

fixes #167384

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[chunhtai]

LGTM