NEP29: Set minimum required version to Python 3.9+

[seisman]

Description of proposed changes

Following NEP29 policy where Python 3.8 will be dropped on or after Apr 14, 2023.

Changes in this PR:

• Update the minimum Python version in ci_tests.yaml
• Update the minimum Python version in ci_tests_legacy.yaml
• Update the minimum Python version in the compatibility table in README
• Update the minimum Python version in doc/install.rst
• 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 #1676 and afed7c9

Fixes #2300.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version