Remove support for Swift 5.1

[Davidde94]

Changes made:

Removed CI config for 5.1
Changed 5.2 to be based on 16.04 so we have some coverage there
Converted Package.swift to the latest syntax
Added a description to supported Swift versions in the README

[Davidde94]

Hey @yim-lee some more CI playing if you don't mind! Could you please:

• Remove the 5.1 jobs
• Update the 5.2 job to use 16.04
• Update the API breakage to be based on the Swift 5.4 Ubuntu 20.04 image

[yim-lee]

@swift-server-bot test this please

[yim-lee]

@swift-server-bot test this please

[yim-lee]

@Davidde94 CI changes are done.

• This project doesn't have api-breakage job :)
• Added 5.4 job to required list
• Fixed 5.5 job
• 5.2 and soundness failures look legit

[Davidde94]

@yim-lee it looks like this is a failure in TSan itself. Could we try swapping the 5.3 job to 16.04, and the 5.2 job to 18.04. I've pushed up the necessary files.

[Davidde94]

@yim-lee could you take another look when you get the chance please?

[yim-lee]

it looks like this is a failure in TSan itself. Could we try swapping the 5.3 job to 16.04, and the 5.2 job to 18.04. I've pushed up the necessary files.

@Davidde94 done.

@swift-server-bot test this please

[Davidde94]

Same TSan failure. For now I'll disable the job, we know that it's unreliable.

[yim-lee]

Same TSan failure. For now I'll disable the job, we know that it's unreliable.

@Davidde94 Are there CI changes you want me to make?

[Davidde94]

@yim-lee I think this is something I can do myself, I'll push up now