doc,CONTRIBUTING.md: update go version to 1.19

[Stavrospanakakis]

Fixes golang/go#53670

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This PR (HEAD: 34611e0) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/pkgsite/+/416114 to see it.

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Patch Set 1:

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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Message from Jonathan Amsterdam:

Patch Set 1: Code-Review+2

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This PR is being closed because golang.org/cl/416114 has been merged.