bump derive-new to 0.6

[fee1-dead]

This will unify syn use to all be 2.0.

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[MichaReiser]

LGTM

What I understand is that the only reason for a new minor version is that syn 2 bumps the minimal required rust version to 1.56. This doesn't apply to Salsa which already requires Rust 1,75

[nikomatsakis]

@fee1-dead seems to have a conflict... perhaps with #505 ?

[fee1-dead]

Resolved.

[nikomatsakis]

Argh, more conflicts.

[nikomatsakis]

Sorry @fee1-dead I didn't merge this earlier, are you up for a rebase?

[nikomatsakis]

Included in #541