gh-38273: Update the deprecation policy
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes #12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes #12345". -->
We update the deprecation policy in the developer guide as proposed in
https://groups.google.com/g/sage-devel/c/7T0Ys01Kcsg
The updated page: https://preview-38273--
sagemath.netlify.app/html/en/developer/coding_in_python#deprecation
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
URL: https://github.com/sagemath/sage/pull/38273
Reported by: Kwankyu Lee
Reviewer(s): Matthias Köppe
