Test with Go 1.19, upgrade golangci-lint (#35)

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.

gitdiff/testdata/apply/bin.go

@@ -1,4 +1,4 @@
-//+build ignore
+//go:build ignore
 
 // bin.go is a helper CLI to manipulate binary diff data for testing purposes.
 // It can decode patches generated by git using the standard parsing functions