[release-17.0] Upgrade go version in upgrade tests to go1.21.9

[frouioui]

Description

This is a follow up of #15639

[vitess-bot]

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[mattlord]

Looks like we need to update the templates and rebuild no?

[frouioui]

Looks like we need to update the templates and rebuild no?

These upgrade/downgrade workflows do not have any template, all manual

[mattlord]

Ok, I missed the part of the title that indicates it was intentional not to change the templates 🙂