Delete simple-ci GitHub Action

[MarcoIeni]

As discussed in Delete simple-ci GitHub Action Zulip stream, we want to remove the simple-ci GitHub Action for the following reasons:

• The action uses node 16 which reached end of life and we don't want to spend time maintaining this action in the future
• As stated in the GitHub Actions readme the action was "supposed to only be used in infra managed by us", but the infra team is not using this action anywhere
• This GitHub search shows that the action is only used in 15 files in GitHub, most of which are markdown files or belong to forks. We want to delete this action to avoid that more repositories adopt it.

Migration guide

Substitute

- uses: rust-lang/simpleinfra/github-actions/simple-ci@master

with

# if checkFmt was true
- name: Install rustfmt
  run: rustup component add rustfmt
- name: Check formatting
  run: cargo fmt --all --check

- name: Build
  run: cargo build --tests --workspace
- name: Test
  run: cargo test --workspace

Plan

I will raise PRs on all the repositories using this action, according to this GitHub search:

• cc1101 ➡️ chore: remove deprecated action dsvensson/cc1101#48
• rustc-hash ➡️ Improve CI with proper test jobs + fmt/doc/clippy rustc-hash#48
• relnotes ➡️ chore: remove deprecated action relnotes#166
• code-cookbook ➡️ chore: remove deprecated action MichaelCurrin/code-cookbook#15
• workflow-builder ➡️ chore: remove deprecated action MichaelCurrin/workflow-builder#1
• rust-project-template ➡️ chore: remove deprecated action MichaelCurrin/rust-project-template#1
• actix-web-docker-example ➡️ chore: remove deprecated action patrick-fitzgerald/actix-web-docker-example#4

The repos with a checkbox are projects we don't want to break. So we want to wait for them to merge the PR before deleting this action. The other projects are just templates or examples, so we can delete the action even if they didn't merge the PR.