internal/imports: update stdlib index for Go 1.21.0

For golang/go#38706.

Change-Id: I278b5edd7caf6ddb0585a3ea8f47777753457ddf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/517075
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Gopher Robot <[email protected]>
Run-TryBot: Gopher Robot <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>

Author: gopherbot