Figure.timestamp: Deprecate parameter 'justification' to 'justify' (remove in v0.13.0)

[seisman]

Description of proposed changes

We use the parameter justify in many modules (e.g., text, plot) but use justification in Figure.timestamp. This PR deprecates the justification parameter to justify to make it consistent in the project.

FYI, the GMT project also use justify as the long option name: https://github.com/GenericMappingTools/gmt/blob/0c3435fba382755a87ba7629cbe1db288c2a9a60/src/gmt_common_longoptions.h#L86

Since it's a rarely used module, I think it's OK to fully remove the deprecated parameter after two releases (i.e., v0.13.0).

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
renamed	pygmt/tests/baseline/test_timestamp_justification.png -> pygmt/tests/baseline/test_timestamp_justify.png

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Report last updated at commit bcb668d

[seisman]

Although Figure.timestamp is rarely used, I think it's our first wrapper which

• has an inline example
• has a gallery example
• has 100% feature completeness (see Add Figure.timestamp to plot the GMT timestamp logo #2208 for context)
• has 100% typing hints completeness
• is Pythonic [arguments are simple strings without modifiers like +t/+o]

So, it's time to give this method a final review, then we can categorize it as "Ongoing maintenance".