_content/tour: fix offline mode go.mod lacking version

[Bertie690]

Previously, an accidentally missing go directive within the generated go.mod file
would effectively set the version to 1.16, which (being quite old) would raise
errors upon attempting to compile generics-related example snippets.

The code has been updated to properly use the latest go version (1.25 as of current).
This may need to be updated in the future if any Go 1.26+-specific examples are added later, but this works for now.

Fixes golang/go#76932

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Patch Set 1:

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This PR (HEAD: c20b2d3) has been imported to Gerrit for code review.

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Patch Set 1:

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