Fix ABI readers not assigning 'platform_name' for GOES-18/19

[djhoese]

Simple fix so that the ABI-based readers know about GOES-18/19. This came up in SIFT development where the data and most of the metadata was perfectly fine, but platform_name was missing (was set to None).

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #1784 (007cf02) into main (983961b) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##             main    #1784   +/-   ##
=======================================
  Coverage   92.90%   92.90%           
=======================================
  Files         265      265           
  Lines       38970    38970           
=======================================
  Hits        36205    36205           
  Misses       2765     2765           

Flag	Coverage Δ
behaviourtests	4.77% <0.00%> (ø)
unittests	93.44% <50.00%> (ø)

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

Impacted Files	Coverage Δ
satpy/readers/abi_base.py	94.59% <50.00%> (ø)

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[coveralls]

Coverage remained the same at 93.387% when pulling 007cf02 on djhoese:bugfix-abi-g18 into 983961b on pytroll:main.

[mraspaud]

Lgtm