upgrade stack to Heroku-20

[LongweiDeng]

Refocus is running in Heroku-16 stack, which will be end-of-life on May 1st, 2021.
We need to upgrade it to Heroku-20.

Ref:
https://help.heroku.com/0S5P41DC/heroku-16-end-of-life-faq
https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack

[salesforce-cla]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Longwei Deng <l***@l***.i***.s***.com>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce.com Contributor License Agreement and this Pull Request will be revalidated.

[coveralls]

Coverage remained the same at 88.708% when pulling d0e6b2b on heroku-stack into 9a23cd1 on master.

[LongweiDeng]

closed due to cla issue