Update Go version to 1.20.6
This resolves [CVE-2023-29406]
(https://nvd.nist.gov/vuln/detail/CVE-2023-29406) for uses of the
`net/http` standard library.
Note that until the follow-up to #18124 is done, the version of Go used
in those impacted tests will need to remain on 1.20.5.
Michael Zalimeni