Figure.grdimage: Deprecate parameter "bit_color" to "bitcolor" (remove in v0.12.0)

[yvonnefroehlich]

Description of proposed changes

To follow the Code style and to be consistent with the similar parameter of pygmt.Figure.image, this PR aims to deprecate the parameter bit_color to bitcolor of pygmt.Figure.grdimage. The FutureWarning warning is currently raised for 2 minor releases.

For context see: #2615 (comment) and #2615 (comment)

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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