Make migration script work with OSX bash and zsh

`sed` has a different in-place command line syntax in FreeBsd
than GNU tools. This change makes the `sed` commands work with
FreeBSD `sed` on OSX in bash and zsh shells.

Issue: androidx/media#217
PiperOrigin-RevId: 522043938

RELEASENOTES.md

@@ -85,6 +85,9 @@
     *   Remove `HlsMasterPlaylist`, use `HlsMultivariantPlaylist` instead.
     *   Remove `Player.stop(boolean)`. Use `Player.stop()` and
         `Player.clearMediaItems()` (if `reset` is `true`) instead.
+*   Migration script
+    *   Fix bug where backup files for each touched file were left in a migrated
+        project on OSX.
 
 ### 1.0.0 (2023-03-22)