Use macos-11 as macos-latest moved to macOS 12.6.2

[EnricoMi]

Checklist before submitting

• Did you read the contributor guide?
• Did you update the docs?
• Did you write any tests to validate this change?
• Did you update the CHANGELOG, if this change affects users?

Description

It looks like installing Python on macOS 12 is broken, it used to work with macOS 11, so pinning tests to macos-11 rather than macos-latest (which recently moved to macos-12).

Review process to land

1. All tests and other checks must succeed.
2. At least one member of the technical steering committee must review and approve.
3. If any member of the technical steering committee requests changes, they must be addressed.

[github-actions]

Unit Test Results

  1 155 files  ±0    1 155 suites  ±0   12h 45m 26s ⏱️ - 7m 54s
     841 tests ±0       788 ✔️ ±0       53 💤 ±0  0 ❌ ±0 
23 809 runs  ±0  17 023 ✔️ ±0  6 786 💤 ±0  0 ❌ ±0 

Results for commit 4c4b898. ± Comparison against base commit 2f52345.

[github-actions]

Unit Test Results (with flaky tests)

  1 282 files  +  28    1 282 suites  +28   13h 19m 22s ⏱️ + 2m 6s
     841 tests ±    0       787 ✔️ ±    0       53 💤 ±    0  1 ❌ ±0 
26 600 runs  +928  18 740 ✔️ +498  7 859 💤 +431  1 ❌  - 1 

For more details on these failures, see this check.

Results for commit 4c4b898. ± Comparison against base commit 2f52345.