Phase out Ubuntu 16.04

[abitmore]

User Story

1. Our documentation (README, Wiki and etc) recommends Ubuntu 16.04 LTS. However, Ubuntu 16.04 LTS will reach its EOL (end of life) in April 2021 which is not too far away. As a user I'd like to be recommended with more up-to-date software.

   If Ubuntu 20.04 LTS support #2216 (Ubuntu 20.04 support) can be done in a timely manner, we could recommend Ubuntu 20.04 directly, which could save us some efforts updating the documentation. Otherwise, we need to update to 18.04 first, then 20.04 or another version later.

• README
• Wiki

2. Ubuntu 16.04 is also used in our CI process -- Travis, docker and etc. We'd better update them as well.

• Travis - closed
• Github Actions - 0a843b5 in Implement alternative black swan response methods #2499
• Docker - moved to 20.04 : Upgrade the Docker base image to Ubuntu 20.04 LTS (Focal Fossa) #2473

3. Release process

• bitshares-gitian - Drop Ubuntu 16.04 bitshares-gitian#49

Impacts
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

• API (the application programming interface)
• Build (the build process or something prior to compiled code)
• CLI (the command line wallet)
• Deployment (the deployment process after building such as Docker, Travis, etc.)
• DEX (the Decentralized EXchange, market engine, etc.)
• P2P (the peer-to-peer network for transaction/block propagation)
• Performance (system or user efficiency, etc.)
• Protocol (the blockchain logic, consensus, validation, etc.)
• Security (the security of system or user data, etc.)
• UX (the User Experience)
• Other (please add below)

Additional Context (optional)
Add any other context about your request here.

CORE TEAM TASK LIST

• Evaluate / Prioritize Feature Request
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation

[abitmore]

Ubuntu 16.04 environment will be removed on September 20, 2021 from Github Actions, see actions/runner-images#3287.

[abitmore]

Done.