Bump Go version to indicate correct minimum requirement (#452)

* Bump Go version to indicate correct minimum requirement

The minimum requirement that we have was silently bumped to 1.21 in #441 because of the `slices` package. It seems that we did not update the `go.mod` when we updated our CI range, because CI did not fail because it was not testing older versions.

We probably should just update the `go.mod` when we update the CI target in the future? Although we could theoretically stay at 1.21 in terms of the code base.

* Removed outdated build tags

* Remove more build tags

* Removed code for Go < 1.20

Author: oxisto

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go: ["1.22", "1.23", "1.24"]
+        go: ["1.21", "1.22", "1.23", "1.24"]
     steps:
       - name: Checkout
         uses: actions/checkout@v4

errors.go

@@ -2,6 +2,7 @@ package jwt
 
 import (
 	"errors"
+	"fmt"
 	"strings"
 )
 
@@ -47,3 +48,42 @@ func joinErrors(errs ...error) error {
 		errs: errs,
 	}
 }
+
+// Unwrap implements the multiple error unwrapping for this error type, which is
+// possible in Go 1.20.
+func (je joinedError) Unwrap() []error {
+	return je.errs
+}
+
+// newError creates a new error message with a detailed error message. The
+// message will be prefixed with the contents of the supplied error type.
+// Additionally, more errors, that provide more context can be supplied which
+// will be appended to the message. This makes use of Go 1.20's possibility to
+// include more than one %w formatting directive in [fmt.Errorf].
+//
+// For example,
+//
+//	newError("no keyfunc was provided", ErrTokenUnverifiable)
+//
+// will produce the error string
+//
+//	"token is unverifiable: no keyfunc was provided"
+func newError(message string, err error, more ...error) error {
+	var format string
+	var args []any
+	if message != "" {
+		format = "%w: %s"
+		args = []any{err, message}
+	} else {
+		format = "%w"
+		args = []any{err}
+	}
+
+	for _, e := range more {
+		format += ": %w"
+		args = append(args, e)
+	}
+
+	err = fmt.Errorf(format, args...)
+	return err
+}

errors_go1_20.go

@@ -1,3 +1,3 @@
 module github.com/golang-jwt/jwt/v5
 
-go 1.18
+go 1.21

errors_go_other.go

@@ -1,6 +1,3 @@
-//go:build go1.4
-// +build go1.4
-
 package jwt
 
 import (

go.mod

@@ -1,6 +1,3 @@
-//go:build go1.4
-// +build go1.4
-
 package jwt_test
 
 import (