Add mention of future deprecations to release script (#19239)

Small improvement to the release script to prompt the user to consider
upcoming deprecations that should be mentioned in the changelog.

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [X] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Olivier 'reivilibre' <oliverw@element.io>

Author: devonh

changelog.d/19239.misc

@@ -0,0 +1 @@
+Prompt user to consider adding future deprecations to the changelog in release script.

scripts-dev/release.py

@@ -291,6 +291,12 @@ def _prepare() -> None:
     synapse_repo.git.add("-u")
     subprocess.run("git diff --cached", shell=True)
 
+    print(
+        "Consider any upcoming platform deprecations that should be mentioned in the changelog. (e.g. upcoming Python, PostgreSQL or SQLite deprecations)"
+    )
+    print(
+        "Platform deprecations should be mentioned at least 1 release prior to being unsupported."
+    )
     if click.confirm("Edit changelog?", default=False):
         click.edit(filename="CHANGES.md")