build: switch to slog Handlers

Start using the new slog handlers. With this commit we also remove the
need for the LogWriter since we let our LogRotator implement io.Writer
and pass that in to our log file handler.

build/log_shutdown.go

@@ -1,7 +1,7 @@
 package build
 
 import (
-	"github.com/btcsuite/btclog"
+	"github.com/btcsuite/btclog/v2"
 )
 
 // ShutdownLogger wraps an existing logger with a shutdown function which will