Update go version to 1.21 in go.mod across all or subset of problems?

[DhanshreeA]

I was solving the vehicle purchase problem and wanted to use the slices package, and the max function for comparing strings, but turns out those are only supported within go 1.21. However the version pinned is go1.18. Some problems use versions even lower than that (go1.16 in hello-world, and lasagna, although admitted these are not the best examples requiring higher versions to begin with)

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