Netlify: select a new build image

[chalin]

As reported via deploy logs:

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DEPRECATION NOTICE: Builds using the Xenial build image will fail after November 15th, 2022.

The build image for this site uses Ubuntu 16.04 Xenial Xerus, which is no longer supported.
All Netlify builds using the Xenial build image will begin failing in the week of November 15th, 2022.

To avoid service disruption, please select a newer build image at the following link:
https://app.netlify.com/sites/grpc-io/settings/deploys#build-image-selection

For more details, visit the build image migration guide:
https://answers.netlify.com/t/please-read-end-of-support-for-xenial-build-image-everything-you-need-to-know/68239
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