CI: Bump to ubuntu-24.04 and mambaforge-23.11 in ReadTheDocs

[weiji14]

Description of proposed changes

Bump ReadTheDocs os version from ubuntu-22.04 to ubuntu-24.04, and mambaforge version from 22.9 to 23.11.

References:

• https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os
• https://docs.readthedocs.io/en/stable/config-file/v2.html#build-tools-python.

Supersedes #3268

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 command is:

• /format: automatically format and lint the code