fix: drop Cirrus CI (going away June 1, 2026)

[henryiii]

Dropping Cirrus CI. The platform is going away June 1, 2026, see https://cirruslabs.org/.

After this PR, cibuildwheel should still work fine on Cirrus for the next couple of months, as we don't have any custom code checking Cirrus CI, other than the ability to detect it specifically.

AI usage disclaimer: created with the prompt "Cirrus CI is shutting down. Let's remove all mentions and files related to Cirrus here" in copilot in vscode, using the claude-haiku-4.5 model (0.33x credit cost, this is pretty easy for AI). (just the code changes, not commit messages / PR description).

I forgot to update main, so there were conflicts, so I used the following prompt and gpt-5-mini (0x credit cost, this is very easy for AI): "Oops, changes were made upstream. Please git rebase on main and fix up conflicts".

I manually ran prek -a since we don't have an AGENTS.md here. There was one mistake from the rebase I fixed (if I asked the model to do it, or we had AGENTS.md, it would have fixed it. Linters are more important than ever with AI).

[henryiii]

Summary from copilot:

Perfect! I've successfully removed all Cirrus CI mentions and files from the cibuildwheel repository. Here's a summary of what was deleted and changed:

Files Deleted:

• .cirrus.yml - Main Cirrus CI configuration
• examples/cirrus-ci-minimal.yml - Example minimal config
• examples/cirrus-ci-intel-mac.yml - Example macOS config
• docs/data/readme_icons/cirrusci.svg - Icon file

Files Modified:

• run_example_ci_configs.py - Removed Cirrus CI service entry
• projects.py - Removed "cirrusci" from icon list
• CI.md - Removed Cirrus CI from platform support table
• test_ios.py - Removed Cirrus CI skip condition
• README.md - Removed from supported CI services and platform table, removed icon reference
• ci-services.md - Removed entire Cirrus CI documentation section
• extra.js - Removed redirect entry
• ci.py - Removed CIProvider.cirrus_ci enum and detection logic
• linux.py - Removed Cirrus CI from error message
• utils.py - Removed Cirrus CI from architecture compatibility check
• test_android.py - Removed Cirrus CI environment check
• configuration.md - Removed Cirrus CI configuration example tab
• projects.yml - Updated schema comment and removed cirrusci from pillow-heif project
• projects.schema.json - Removed "cirrusci" from enum
• working-examples.md - Removed Cirrus CI icon reference and icon definition

The changelog.md file retains historical entries for reference, which appropriately documents when Cirrus CI support was added to the project.

Made changes.

[agriyakhetarpal]

¡Hasta la vista!

Approving, modulo #2817 (comment).