Bump src-cli MinimumVersion to 3.21.0

Created by: mrnugget

When downloading the latest version of src-cli from sourcegraph.com this version here is used. And 3.20.0 is not the latest one.

We should default to immediately updating this when doing a release in src-cli. I'll add it to the src-cli docs.

(Issue in which a user ran into this: https://github.com/sourcegraph/src-cli/issues/332)