Recommend Figure.timestamp and remove timestamp (U) alias from all plotting methods

[seisman]

Description of proposed changes

Similar to #2071.

The purpose of this PR is to remove timestamp (U) alias and recommend use Figure.timestamp instead.

NOTE: Figure.timestamp is not implemented yet.

Fixes #

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

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specific operations. Supported slash commands are:

• /format: automatically format and lint the code
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[seisman]

The warning message looks like this:

>>> import pygmt
>>> fig = pygmt.Figure()
>>> fig.basemap(region=[0, 10, 0, 10], projection="X10c/10c", frame=True, U=True)
<ipython-input-3-47b2c09a10fd>:1: SyntaxWarning: Parameter 'U' and 'timestamp' is deprecated since v0.8.0. Use Figure.timestamp() instead.
  fig.basemap(region=[0, 10, 0, 10], projection="X10c/10c", frame=True, U=True)

>>> fig.show()

>>> import pygmt
>>> fig = pygmt.Figure()
>>> fig.basemap(region=[0, 10, 0, 10], projection="X10c/10c", frame=True, timestamp=True)
<ipython-input-7-eff609310ed5>:1: SyntaxWarning: Parameter 'U' and 'timestamp' is deprecated since v0.8.0. Use Figure.timestamp() instead.
  fig.basemap(region=[0, 10, 0, 10], projection="X10c/10c", frame=True, timestamp=True)
>>> fig.show()

[weiji14]

NOTE: Figure.timestamp is not implemented yet.

Will definitely need to implement this before this PR can be approved 🙂

[michaelgrund]

Looks good!