GOATS-550: Fix issue plotting GHOST data.

[davner]

• Modify grabbing DATE-OBS and TIME-OBS from header.
• Now cleanly handle missing TIME-OBS or DATE-OBS and return default UTC now if missing.

Checklist

• Added a release note in doc/changes using the PR number.

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.26%. Comparing base (afffb53) to head (0cac8fa).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/goats_tom/facilities/gemini.py	55.55%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #405      +/-   ##
==========================================
- Coverage   76.29%   76.26%   -0.03%     
==========================================
  Files         232      232              
  Lines        7331     7335       +4     
  Branches      455      455              
==========================================
+ Hits         5593     5594       +1     
- Misses       1654     1657       +3     
  Partials       84       84              

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