feat: go 1.22 and slog migration (#557)

This change adds Go 1.22 as a build target and drops support for Go 1.20 and older. The golang.org/x/exp/slog import is migrated to log/slog.

Slog has been part of the Go standard library since Go 1.21. Therefore we are dropping support for older Go versions. This is in line of our support policy of "the latest two Go versions".

pkg/op/op.go

@@ -3,6 +3,7 @@ package op
 import (
 	"context"
 	"fmt"
+	"log/slog"
 	"net/http"
 	"time"
 
@@ -12,7 +13,6 @@ import (
 	"github.com/zitadel/schema"
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/trace"
-	"golang.org/x/exp/slog"
 	"golang.org/x/text/language"
 
 	httphelper "github.com/zitadel/oidc/v3/pkg/http"
@@ -114,8 +114,6 @@ type OpenIDProvider interface {
 	Crypto() Crypto
 	DefaultLogoutRedirectURI() string
 	Probes() []ProbesFn
-
-	// EXPERIMENTAL: Will change to log/slog import after we drop support for Go 1.20
 	Logger() *slog.Logger
 
 	// Deprecated: Provider now implements http.Handler directly.
@@ -646,8 +644,6 @@ func WithCORSOptions(opts *cors.Options) Option {
 }
 
 // WithLogger lets a logger other than slog.Default().
-//
-// EXPERIMENTAL: Will change to log/slog import after we drop support for Go 1.20
 func WithLogger(logger *slog.Logger) Option {
 	return func(o *Provider) error {
 		o.logger = logger