Migration: stop recommending --prereleases with pub outdated --mode=null-safety.

As of
dart-lang/pub@5fba201,
`--mode=null-safety` implies `--prereleases`.

Change-Id: I34bc827ddd3579e968276f826c0420ffadc0eac3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/171661
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Commit-Queue: Devon Carew <[email protected]>
Reviewed-by: Devon Carew <[email protected]>

Author: stereotype441

pkg/nnbd_migration/lib/src/messages.dart

@@ -34,7 +34,7 @@ are currently importing the following non-null-safe libraries:
 
 Please upgrade the packages containing these libraries to null safe versions
 before continuing.  To see what null safe package versions are available, run
-the following command: `dart pub outdated --mode=null-safety --prereleases`.
+the following command: `dart pub outdated --mode=null-safety`.
 
 To skip this check and try to migrate anyway, re-run with the flag
 `$_skipImportCheckFlag`.