Generate a buildcaptain rotation for 2021

Create a buildcaptain rotation csv starting on jan 1 2021
using the same rotation of names that we were using at end
of december.

Removed all the rotation overrides from 2020.

Here's the command I ran to generate the rotation:

```bash
go run ./cmd/generate-rotation-csv/main.go \
  -overrides ./rotation-overrides.csv \
  -start-date 2021-01-01 \
  -days 365 \
  -names wlynch,dibyo,nikthoma,sashture,sbws,jerop,vdemeest,andrea.frittoli \
  > ./rotation.csv
```

bots/buildcaptain/rotation-overrides.csv

@@ -1,11 +1,2 @@
 Date,User
-2020-05-25,,Happy holidays!
-2020-12-24,,Happy holidays!
-2020-12-25,,Happy holidays!
-2020-12-26,,Happy holidays!
-2020-12-27,,Happy holidays!
-2020-12-28,,Happy holidays!
-2020-12-29,,Happy holidays!
-2020-12-30,,Happy holidays!
-2020-12-31,,Happy holidays!
 2021-1-1,,Happy holidays!