all: replace log15 with slog (#28187)

This PR replaces Geth's logger package (a fork of [log15](https://github.com/inconshreveable/log15)) with an implementation using slog, a logging library included as part of the Go standard library as of Go1.21.

Main changes are as follows:
* removes any log handlers that were unused in the Geth codebase.
* Json, logfmt, and terminal formatters are now slog handlers.
* Verbosity level constants are changed to match slog constant values.  Internal translation is done to make this opaque to the user and backwards compatible with existing `--verbosity` and `--vmodule` options.
* `--log.backtraceat` and `--log.debug` are removed.

The external-facing API is largely the same as the existing Geth logger.  Logger method signatures remain unchanged.

A small semantic difference is that a `Handler` can only be set once per `Logger` and not changed dynamically.  This just means that a new logger must be instantiated every time the handler of the root logger is changed.

----
For users of the `go-ethereum/log` module. If you were using this module for your own project, you will need to change the initialization. If you previously did 
```golang
log.Root().SetHandler(log.LvlFilterHandler(log.LvlInfo, log.StreamHandler(os.Stderr, log.TerminalFormat(true))))
```
You now instead need to do 
```golang
log.SetDefault(log.NewLogger(log.NewTerminalHandlerWithLevel(os.Stderr, log.LevelInfo, true)))
```
See more about reasoning here: https://github.com/ethereum/go-ethereum/issues/28558#issuecomment-1820606613

cmd/bootnode/main.go

@@ -32,6 +32,7 @@ import (
 	"github.com/ethereum/go-ethereum/p2p/enode"
 	"github.com/ethereum/go-ethereum/p2p/nat"
 	"github.com/ethereum/go-ethereum/p2p/netutil"
+	"golang.org/x/exp/slog"
 )
 
 func main() {
@@ -52,10 +53,10 @@ func main() {
 	)
 	flag.Parse()
 
-	glogger := log.NewGlogHandler(log.StreamHandler(os.Stderr, log.TerminalFormat(false)))
-	glogger.Verbosity(log.Lvl(*verbosity))
+	glogger := log.NewGlogHandler(log.NewTerminalHandler(os.Stderr, false))
+	glogger.Verbosity(slog.Level(*verbosity))
 	glogger.Vmodule(*vmodule)
-	log.Root().SetHandler(glogger)
+	log.SetDefault(log.NewLogger(glogger))
 
 	natm, err := nat.Parse(*natdesc)
 	if err != nil {