WIP: SPEC 0: Bump minimum supported version to Python 3.11

[seisman]

Description of proposed changes

Following SPEC 0 policy where Python 3.10 should be dropped in 2024 quarter 4.

Changes in this PR:

• [x] Update the minimum Python version in ci_tests.yaml
• [x] Update the minimum Python version in ci_tests_legacy.yaml
• [x] Update the Python version in the requires-python field and remove the related entry from classifiers in pyproject.toml.
• [ ] Update branch protection rules

Supersedes PR #3039.

Need to wait for #3490 to support Python 3.13 first.