Update Go and golangci-lint versions (#49)
The minimum Go version for the package is now Go 1.21. This is because a
future change will use the 'slices' package in test code. Note that non-test
code in the package should still be compatible with older versions of Go.
As part of this, also update the golangci-lint version to one that works
with Go 1.21, which required replacing some deprecated linters.
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