Remove mention of fallback to GDAL in geotiff writer

[djhoese]

This bad information came up in the monthly meeting we had today. The geotiff writer does not use GDAL's python bindings directly at all anymore. This PR removes the mention of this and states that rasterio is required.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2200 (913d27a) into main (464d845) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2200   +/-   ##
=======================================
  Coverage   94.07%   94.07%           
=======================================
  Files         290      290           
  Lines       44667    44667           
=======================================
  Hits        42021    42021           
  Misses       2646     2646           

Flag	Coverage Δ
behaviourtests	4.72% <ø> (ø)
unittests	94.73% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/writers/geotiff.py	93.75% <ø> (ø)

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[coveralls]

Coverage remained the same at 94.682% when pulling 913d27a on doc-geotiff-gdalfallback into 464d845 on main.

[djhoese]

Got approval on slack.