chore: bump version to 6.4.3 w/ changelog

[jelbourn]

No description provided.

[josephperrott]

LGTM

[josephperrott]

674ebcd also needs to be cherry picked into the branch

[googlebot]

A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)

[googlebot]

CLAs look good, thanks!

[jelbourn]

@josephperrott done

[ngbot]

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