Update deprecated -JG syntax

[michaelgrund]

Description of proposed changes

This PR updates the deprecated -JG syntax to account for GMT 6.3 change made at GenericMappingTools/gmt#5780.

Preview at https://pygmt-git-update-depr-jg-gmt.vercel.app/projections/azim/azim_general_perspective.html

Part of #1644

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.

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

[weiji14]

Looks alright to me. The new syntax does look nicer, will save people from counting all the slashes to figure out which number is changing what parameter!