Bug: Made it so that 0 was included in uint8
- [ ] closes #14401 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Decided to restart. Sorry for the inconvenience. -> #14472

Thx for the PR. pls rebase existing PR from next.
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14412?src=pr) is 85.26% (diff: 100%)

> Merging [#14412](https://codecov.io/gh/pandas-dev/pandas/pull/14412?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14412   diff @@
==========================================
  Files           140        140          
  Lines         50631      50634     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43167      43173     +6   
+ Misses         7464       7461     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [7cad3f1...2b2622c](https://codecov.io/gh/pandas-dev/pandas/compare/7cad3f16bccd1d4702ef9d038b1ee0db33b9bb94...2b2622c25b80e41ea1cdcb44d439f930c81c7abc?src=pr)
 Added a series of tests which applied a pandas series that included the minimum and maximum values of each dtype to a to_numeric downcast to see if it would downcast to the appropriate dtype. It did not add to uint64 but I'm not sure if there were plans to include uint64 so I'll open up a new issue. Also edited the whatsnew.
