NEP29: Set minimum required version to NumPy 1.21+

[weiji14]

Description of proposed changes

Following NEP29 policy where NumPy 1.20 is to be dropped on or after 31 January 2023 (in a minor version increment, i.e. for PyGMT v0.9.0). Bumps minimum supported NumPy version to 1.21 in the pyproject.toml, requirements.txt and environment.yml files. Also update installation documentation to mention NumPy 1.21+ requirement.

This is in line with PyGMT's policy on NEP29 at https://www.pygmt.org/v0.8.0/maintenance.html#dependencies-policy, xref #1074.

Note that CI tests already run on NumPy 1.21 since #2057.

Supersedes #1985.

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