zapslog: Delete x/exp-based implementation

Deletes the implementation of slog.Handler based on
golang.org/x/exp/slog.
Maintaining two implementations doesn't serve much value here.

Remaining files have been renamed to better match their contents:

- handler.go holds the slog.Handler
- handler_test.go tests that
- example_test.go contains example tests

Files that remain are still tagged with `go1.21`
so they're only visible to Go 1.21 or newer.

This way, `go test` in this directory with Go 1.20 will not fail:

```
% GOTOOLCHAIN=go1.20 go test -v
?       go.uber.org/zap/exp/zapslog     [no test files]
```

Resolves uber-go#1337

exp/zapslog/doc.go

@@ -21,6 +21,5 @@
 // Package zapslog provides an implementation of slog.Handler which writes to
 // the supplied zapcore.Core.
 //
-// For versions of Go before 1.21, this package uses golang.org/x/exp/slog.
-// For Go 1.21 or newer, this package uses the standard log/slog package.
+// Use of this package requires at least Go 1.21.
 package zapslog // import "go.uber.org/zap/exp/zapslog"

exp/zapslog/slog_go121_test.go → exp/zapslog/handler_test.go

@@ -43,7 +43,7 @@ func TestAddSource(t *testing.T) {
 	entry := logs.AllUntimed()[0]
 	r.Equal("msg", entry.Message, "Unexpected message")
 	r.Regexp(
-		`/slog_go121_test.go:\d+$`,
+		`/handler_test.go:\d+$`,
 		entry.Caller.String(),
 		"Unexpected caller annotation.",
 	)