Update AHI gridded reader to use HTTP instead of FTP

[simonrp84]

The AHI gridded reader currently uses FTP to downlod the required look up tables. This PR switches to HTTP.

[codecov]

Codecov Report

Merging #1897 (bb4b2fe) into main (ef301b0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1897   +/-   ##
=======================================
  Coverage   93.29%   93.30%           
=======================================
  Files         273      273           
  Lines       40460    40464    +4     
=======================================
+ Hits        37748    37754    +6     
+ Misses       2712     2710    -2     

Flag	Coverage Δ
behaviourtests	4.71% <0.00%> (-0.01%)	⬇️
unittests	93.86% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
satpy/readers/ahi_l1b_gridded_bin.py	100.00% <100.00%> (+0.94%)	⬆️
...tpy/tests/reader_tests/test_ahi_l1b_gridded_bin.py	100.00% <100.00%> (+0.65%)	⬆️

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[coveralls]

Coverage increased (+0.006%) to 93.788% when pulling bb4b2fe on simonrp84:ahi_grid_fix into ef301b0 on pytroll:main.

[mraspaud]

LGTM