DOC: attempt to fix contributors for 0.24.0, 0.25.0/1/2
This hopefully addresses https://github.com/pandas-dev/pandas/issues/29151, please review!

CC @TomAugspurger @jreback 
lgtm. can you have a look at the other whatsnew for the same issue? Looks like there are a few

```
doc/source/whatsnew/v0.24.0.rst:1936:.. contributors:: v0.23.4..HEAD
doc/source/whatsnew/v0.25.1.rst:119:.. contributors:: v0.25.0..HEAD
doc/source/whatsnew/v0.25.0.rst:1270:.. contributors:: v0.24.x..HEAD
doc/source/whatsnew/v0.25.2.rst:49:.. contributors:: v0.25.1..HEAD
```

I think all of those can be updated. Okay, the `.x` technique only works with branches i.e. w/o the `v` prefix, and even then the underlying git command might fail with

```
$ git shortlog -s 0.24.x..v0.25.0
fatal: ambiguous argument '0.24.x..v0.25.0': unknown revision or path not in the working tree.
```

Not sure how the repo is cloned / checked out in the environment used for building documentation. But in my clone of the repo I need to do
```
$ git shortlog -s origin/0.24.x..v0.25.0
```
to make this technique work.

Edit: so, either we use tags as  in `v0.24.2..v0.25.0` which seems to work in every build environment or we use a combination of branch names and tags as in `origin/0.24.x..v0.25.0` which seems to depend on the checkout (or at least my git knowledge is not good enough to understand the requirements about the checkout here).

Going to adjust the PR to only use tags (using the _latest_ as of today, i.e. 0.23.4 and 0.24.2 as the base). Thanks!