TiChi bot domain migration

#1

New domain for TiChi bot is prow.tidb.net, it was online in 2022 September.
The old domain(prow.tidb.io) will be down at 2023-01-01.

Thanks.

#2

prow.tidb.io was offline
Please visit to https://prow.tidb.net

Ref: https://github.com/ti-community-infra/configs/pull/784