[NET-5574] Update Go version to 1.20.8

[zalimeni]

This brings us up to the latest Go version and resolves several CVEs:

• CVE-2023-39320: cmd/go: go.mod toolchain directive allows arbitrary execution
• CVE-2023-39318: html/template: improper handling of HTML-like comments within script contexts
• CVE-2023-39319: html/template: improper handling of special tags within script contexts
• CVE-2023-39321 and CVE-2023-39322: crypto/tls: panic when processing post-handshake message on QUIC connections

Note that 1.0.x uses Go 1.19, so a separate PR will "backport" this change there.

How I've tested this PR: CI

How I expect reviewers to test this PR: 👀

Checklist:

• Tests added
• CHANGELOG entry added

[zalimeni]

FYI, acceptance tests for Vault are currently broken due to a faulty test, and should not be affected by this change.

Rebased to include test fix.