Move pygmt.show_versions function to _show_versions.py

[weiji14]

Description of proposed changes

Move pygmt.show_versions() function from __init__.py to _show_versions.py because it got too long after #3244, and ruff v0.4.6 introduced a minor change in astral-sh/ruff#11521 that counts every branch in the match statement as a single statement, causing pygmt.show_versions() to exceed the 50 statement limit.

Reference:

• https://docs.astral.sh/ruff/rules/too-many-statements

Fixes #3276

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.

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