Add deprecation warnings for add_bounds boolean args

[tomvothecoder]

Description

• Closes [Bug]: Add deprecation warning if users pass add_bounds=True to open_dataset/open_mfdataset #546

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6b45211) 100.00% compared to head (d495499) 100.00%.

❗ Current head d495499 differs from pull request most recent head 73e6851. Consider uploading reports for the commit 73e6851 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #548   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines         1592      1598    +6     
=========================================
+ Hits          1592      1598    +6     

Files Changed	Coverage Δ
xcdat/dataset.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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