Phase out Ubuntu 18.04 LTS (Bionic Beaver)

[abitmore]

User Story

As a software project maintainer I want software that will soon reach its EOL is not used during development so that we are not affected by outdated software.

Additional Context (optional)
Ubuntu Linux 18.04 LTS will reach the end of its hardware and maintenance update LTS support window on April 30, 2023.

Github Actions announced deprecation of The Ubuntu 18.04 Actions runner image.

We have started the deprecation process for Ubuntu 18.04. While the image is being deprecated, You may experience longer queue times during peak usage hours. Deprecation will begin on 2022/08/08 and the image will be fully unsupported by 2023/04/01

Related (to learn what to do): #2217

• Github Actions
  • bitshares-core: Use ElasticSearch 8.5.3 in Github Actions, remove Ubuntu 18 #2707
  • bitshares-fc: bitshares/bitshares-fc@007ed0e in Improve safe<T> bitshares-fc#248, and 11b2a81 in Use gcovr to process code coverage data #2739
• bitshares-gitian - Build all BitShares-Core binaries in Ubuntu 20.04, and bump library versions bitshares-gitian#72
• Docker - Done in version 6.0
• README and Wiki - Done in version 6.0

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)

CORE TEAM TASK LIST

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

[abitmore]

Done.