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/utils/flags.go

@@ -1389,6 +1389,13 @@ func SetNodeConfig(ctx *cli.Context, cfg *node.Config) {
 		log.Info(fmt.Sprintf("Using %s as db engine", dbEngine))
 		cfg.DBEngine = dbEngine
 	}
+	// deprecation notice for log debug flags (TODO: find a more appropriate place to put these?)
+	if ctx.IsSet(LogBacktraceAtFlag.Name) {
+		log.Warn("log.backtrace flag is deprecated")
+	}
+	if ctx.IsSet(LogDebugFlag.Name) {
+		log.Warn("log.debug flag is deprecated")
+	}
 }
 
 func setSmartCard(ctx *cli.Context, cfg *node.Config) {

cmd/utils/flags_legacy.go

@@ -45,6 +45,8 @@ var DeprecatedFlags = []cli.Flag{
 	LightMaxPeersFlag,
 	LightNoPruneFlag,
 	LightNoSyncServeFlag,
+	LogBacktraceAtFlag,
+	LogDebugFlag,
 }
 
 var (
@@ -118,6 +120,18 @@ var (
 		Usage:    "Enables serving light clients before syncing (deprecated)",
 		Category: flags.LightCategory,
 	}
+	// Deprecated November 2023
+	LogBacktraceAtFlag = &cli.StringFlag{
+		Name:     "log.backtrace",
+		Usage:    "Request a stack trace at a specific logging statement (deprecated)",
+		Value:    "",
+		Category: flags.DeprecatedCategory,
+	}
+	LogDebugFlag = &cli.BoolFlag{
+		Name:     "log.debug",
+		Usage:    "Prepends log messages with call-site location (deprecated)",
+		Category: flags.DeprecatedCategory,
+	}
 )
 
 // showDeprecated displays deprecated flags that will be soon removed from the codebase.