build: modernize Go usage

This commit modernizes Go usage. This was done with:

go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

Then files needed to be `go fmt`ed and a few comments needed to be
restored.

The modernizations include replacing

- if/else conditional assignment by a call to the built-in min or max functions added in go1.21
- sort.Slice(x, func(i, j int) bool) { return s[i] < s[j] } by a call to slices.Sort(s), added in go1.21
- interface{} by the 'any' type added in go1.18
- append([]T(nil), s...) by slices.Clone(s) or slices.Concat(s), added in go1.21
- loop around an m[k]=v map update by a call to one of the Collect, Copy, Clone, or Insert functions from the maps package, added in go1.21
- []byte(fmt.Sprintf...) by fmt.Appendf(nil, ...), added in go1.19
- append(s[:i], s[i+1]...) by slices.Delete(s, i, i+1), added in go1.21
- a 3-clause for i := 0; i < n; i++ {} loop by for i := range n {}, added in go1.22

backend/sftp/ssh_external.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"os/exec"
+	"slices"
 	"strings"
 	"time"
 
@@ -89,7 +90,7 @@ func (f *Fs) newSSHSessionExternal() *sshSessionExternal {
 	// Connect to a remote host and request the sftp subsystem via
 	// the 'ssh' command. This assumes that passwordless login is
 	// correctly configured.
-	ssh := append([]string(nil), s.f.opt.SSH...)
+	ssh := slices.Clone(s.f.opt.SSH)
 	s.cmd = exec.CommandContext(ctx, ssh[0], ssh[1:]...)
 
 	// Allow the command a short time only to shut down