Fix ABI rayleigh_corrected_crefl modifier using deprecated DEM specifier

[djhoese]

Forgot to fix this in #2015. The ABI crefl modifier wasn't updated when I changed how the CMGDEM.hdf files are downloaded/loaded by the modifier. It should point to a URL that can get downloaded and not a non-existent file.

• Closes #xxxx
• Tests added

[codecov]

Codecov Report

Merging #2017 (870d287) into main (9e17b74) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2017      +/-   ##
==========================================
- Coverage   93.56%   93.56%   -0.01%     
==========================================
  Files         282      282              
  Lines       41637    41637              
==========================================
- Hits        38958    38957       -1     
- Misses       2679     2680       +1     

Flag	Coverage Δ
behaviourtests	4.78% <ø> (ø)
unittests	94.09% <ø> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
satpy/modifiers/_crefl.py	93.02% <0.00%> (-1.17%)	⬇️

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[coveralls]

Coverage decreased (-0.002%) to 94.021% when pulling 870d287 on djhoese:bugfix-abi-crefl-height into 9e17b74 on pytroll:main.