Optimizations: Only link used datasets, avoid copies, avoid pkg/errors dependency

[mologie]

This PR contains various minor optimizations that aim to reduce the overall binary size.

Size/complexity optimizations:

• Load datasets into individual byte slices, which allows the Go linker to strip out unused ones. This reduces binary size by 10+MB when using only the Provinces10 dataset.
• Remove github.com/pkg/errors in favor of Go's native error wrapping

Misc cosmetics:

• Avoid copying data (gzip, slices) where possible, showed up on a memory profile but had no practical impact
• Use Go 1.21 in CI because 1.19 is EOL

[sams96]

Sorry it's taken me a while to get to this, but it all looks good thank you very much.