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

lib/jwtutil/jwtutil.go

@@ -19,6 +19,8 @@ import (
 	"github.com/rclone/rclone/fs/config/configmap"
 	"github.com/rclone/rclone/lib/oauthutil"
 
+	"maps"
+
 	"golang.org/x/oauth2"
 )
 
@@ -32,11 +34,9 @@ func RandomHex(n int) (string, error) {
 }
 
 // Config configures rclone using JWT
-func Config(id, name, url string, claims jwt.Claims, headerParams map[string]interface{}, queryParams map[string]string, privateKey *rsa.PrivateKey, m configmap.Mapper, client *http.Client) (err error) {
+func Config(id, name, url string, claims jwt.Claims, headerParams map[string]any, queryParams map[string]string, privateKey *rsa.PrivateKey, m configmap.Mapper, client *http.Client) (err error) {
 	jwtToken := jwt.NewWithClaims(jwt.SigningMethodRS256, claims)
-	for key, value := range headerParams {
-		jwtToken.Header[key] = value
-	}
+	maps.Copy(jwtToken.Header, headerParams)
 	payload, err := jwtToken.SignedString(privateKey)
 	if err != nil {
 		return fmt.Errorf("jwtutil: failed to encode payload: %w", err)