Change the vintage cursor height number box to a slider.
## References
Related: #9370
## PR Checklist
* [x] Closes#9377
* [x] zadjii-msft edit: Now _this one_ closes#9175
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Schema updated.
* [ ]
## Detailed Description of the Pull Request / Additional comments
It seems like the cursor height couldn't be lower than 25 percent regardless of the given value, so I've changed the `MinCursorHeightPercent` in CustomTextRenderer header file.
## Validation Steps Performed
Manual validation
![CursorHeightSlider](https://user-images.githubusercontent.com/39456018/110041939-bf076080-7d66-11eb-8d58-ba9a84922803.gif)