Update ml-metadata version

[chr12c]

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Update version of ml-metadata used in the kfp provider service (currently using v1.5.0). The most recent version today is v1.16.0.

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Unsure what the latest compatible version might be, but at the very least v1.6.0 looks to be ok judging from the release notes.

Additional context
This comes as a result of the change to introduce the provider service. No updates to the version of ml-metadata was made in the PR to decrease the amount of variables.