Fix bug in `Series.describe` where the median is included any time the `percentiles` argument is not None
- [x] closes #60550 (Replace xxxx with the GitHub issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This PR aims at fixing the bug mentioned in the issue referenced above. Multiple PRs (#61024, #61023 , #60986 , #60557) have already attempted to resolve it, yet all of them got closed or stale for more than 2 weeks. So I allowed myself to apply here all the comments in the issue and previous PRs.

One last thing left hanging pertains to backward incompatibility. The only case in which the result will change is when `percentiles` is a list that does not contain 0.5. Before the PR, it adds 0.5 to the result; after the PR, it does not include it. 
If we add a warning message in such case, it would be considered as:
* useful for the people who read the median even though they didn't include it in the list of percentiles
* spamming / undesirable for the rest

So, what is pandas' policy regarding warning messages and its potentially large number of false alerts?

@mroeschke Thanks for the feedback! I applied your comments. Lmk if there's anything else to update. Thanks @MartinBraquet 