Update old date_planner README.md (#2598)

*Replace this paragraph with a description of what this PR is changing
or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. For larger changes, raising an
issue first helps
reduce redundant work.*

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).


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<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md
[changelog]: ../CHANGELOG.md

experimental/date_planner/README.md

@@ -1,4 +1,3 @@
-# Sample retired
+# Relocation notice
 
-The date-planner app has been retired. For examples of iOS and 
-Cupertino-style applications, see the [rolodex][../rolodex/] app.
+The date-planner app has been moved [to the samples directory](../../date_planner).