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

eth/catalyst/api_test.go

@@ -1562,7 +1562,7 @@ func TestBlockToPayloadWithBlobs(t *testing.T) {
 
 // This checks that beaconRoot is applied to the state from the engine API.
 func TestParentBeaconBlockRoot(t *testing.T) {
-	log.Root().SetHandler(log.LvlFilterHandler(log.LvlTrace, log.StreamHandler(colorable.NewColorableStderr(), log.TerminalFormat(true))))
+	log.SetDefault(log.NewLogger(log.NewTerminalHandlerWithLevel(colorable.NewColorableStderr(), log.LevelTrace, true)))
 
 	genesis, blocks := generateMergeChain(10, true)
 

eth/downloader/queue_test.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"math/big"
 	"math/rand"
+	"os"
 	"sync"
 	"testing"
 	"time"
@@ -31,6 +32,7 @@ import (
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/params"
 	"github.com/ethereum/go-ethereum/trie"
+	"golang.org/x/exp/slog"
 )
 
 // makeChain creates a chain of n blocks starting at and including parent.
@@ -271,7 +273,7 @@ func XTestDelivery(t *testing.T) {
 	world.chain = blo
 	world.progress(10)
 	if false {
-		log.Root().SetHandler(log.StdoutHandler)
+		log.SetDefault(log.NewLogger(slog.NewTextHandler(os.Stdout, nil)))
 	}
 	q := newQueue(10, 10)
 	var wg sync.WaitGroup