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.
The previous version of the golangci-lint action would install its own
version of Go, which eventually conflicted with the old pinned version
of the linter I was using. Upgrade the action to avoid this, but also
update Go and the linter while I'm here.
Use golangci-lint to coordinate linting instead of installing tools
individually. The workflow installs the binary directly instead of using
one of the existing public actions.
Billy Keyes