Replace deprecated plot_date with plot

[m4cd4r4]

Fixes #603.

ax.plot_date() was deprecated in matplotlib 3.9. Replaced with ax.plot() in plot_airmass, plot_altitude, and plot_parallactic. Added ax.xaxis_date() before each formatting block so the axis still uses date-aware locating.

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.08%. Comparing base (a4d89a0) to head (d74a38e).

Files with missing lines	Patch %	Lines
astroplan/plots/time_dependent.py	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #636      +/-   ##
==========================================
- Coverage   72.90%   72.08%   -0.82%     
==========================================
  Files          14       14              
  Lines        1849     1845       -4     
==========================================
- Hits         1348     1330      -18     
- Misses        501      515      +14     

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