Update dependencies to newer stable version

[tok-kkk]

Object

Some of our dependencies haven't been updated for a long time.
We should update them to newer stable version to include those bug fixes and performance improvement.

Design

• We use v1 of urfave/cli since v2 was still in beta at the time we started. v2 is the recommended version than v1. We should migrate our code from v1 to v2.
• The latest go-ethereum stable version is v1.9.15. This also upgrades the minimum go version to 1.13 which we should put in our docs.
• Remove unused dependencies and clean up the go module

Implementation

• Follow the migration guide to upgrade urfave/cli to v2
• Upgrade go-ethereum to 1.9.15 and update minimum required go version
• Run go mod tidy to clean up our go module