[release-18.0] Update go.mod go version to 1.21.9

[shlomi-noach]

Description

Followup to #15639, we were missing the updated go version in go.mod.

Related Issue(s)

#15639

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[vitess-bot]

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[frouioui]

For future references, I don't think we really need to upgrade the minor version of go.mod with go1.21. @shlomi-noach were you seeing errors without it?

[shlomi-noach]

I was not seeing errors without it, but the inconsistency led me to believe there was an oversight. For consistency alone I think it's worth keeping the version fully in sync.