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

internal/debug/api.go

@@ -37,6 +37,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/hashicorp/go-bexpr"
+	"golang.org/x/exp/slog"
 )
 
 // Handler is the global debugging handler.
@@ -56,7 +57,7 @@ type HandlerT struct {
 // Verbosity sets the log verbosity ceiling. The verbosity of individual packages
 // and source files can be raised using Vmodule.
 func (*HandlerT) Verbosity(level int) {
-	glogger.Verbosity(log.Lvl(level))
+	glogger.Verbosity(slog.Level(level))
 }
 
 // Vmodule sets the log verbosity pattern. See package log for details on the
@@ -65,12 +66,6 @@ func (*HandlerT) Vmodule(pattern string) error {
 	return glogger.Vmodule(pattern)
 }
 
-// BacktraceAt sets the log backtrace location. See package log for details on
-// the pattern syntax.
-func (*HandlerT) BacktraceAt(location string) error {
-	return glogger.BacktraceAt(location)
-}
-
 // MemStats returns detailed runtime memory statistics.
 func (*HandlerT) MemStats() *runtime.MemStats {
 	s := new(runtime.MemStats)