[CI] switch from Travis to Github Action

[johnnychen94]

Changes:

• remove appveyor (and also windows 32bit)
• leave travis for Documentation and Coverage coverage report and documentation are moved to Github Action, too.
• caching artifacts dir in CI

[johnnychen94]

AppVeyor, however, needs to be uninstalled after this. Otherwise, it will give a big ❌ in the CI status.

[kimikage]

I will merge this around 12:00 pm UTC today (July 18th). However, I do not have the permission to remove AppVeyor (webhook).

[kimikage]

Thanks!

We already have multiple commits on gh-pages, so the warning below seems to be ignorable.
https://github.com/JuliaGraphics/Colors.jl/runs/884894599?check_suite_focus=true#step:6:21

[johnnychen94]

FYI, to tell TagBot not include issues/PRs in release summary, you can tag them with no changelog

https://github.com/JuliaRegistries/TagBot
By default, the following labels are ignored:

• changelog skip
• duplicate
• exclude from changelog
• invalid
• no changelog
• question
• wont fix

[kimikage]

Personally I don't feel the need to exclude this, though.:smile:

[johnnychen94]

Heads up: TagBot needs a ssh key to trigger the docs CI when there's a new release.

Check the "TagBot & tagged versions" warning for more details: https://juliadocs.github.io/Documenter.jl/dev/man/hosting/#GitHub-Actions