Stop testing on `macos-11`

[dentarg]

test (macos-11, 3.1)
A brownout will take place on June 24 2024, 3:00 AM – 9:00 PM EST to raise awareness of the upcoming macOS-11 environment removal.

[dentarg]

From https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

The macos-11 label has been deprecated and will no longer be available after 28 June 2024.

[dentarg]

https://github.blog/changelog/2024-05-20-actions-upcoming-changes-to-github-hosted-macos-runners/