Switched Mailgun SDK to `mailgun.js`

[daniellockyer]

refs https://github.com/TryGhost/Toolbox/issues/363

• this commit switches us to using the official and maintained mailgun.js SDK

[codecov[bot]]

Codecov Report

Merging #15210 (651904e) into main (0a68ea8) will increase coverage by 0.01%. The diff coverage is 79.16%.

@@            Coverage Diff             @@
##             main   #15210      +/-   ##
==========================================
+ Coverage   52.81%   52.82%   +0.01%     
==========================================
  Files        1359     1359              
  Lines       86981    86979       -2     
  Branches     9810     9810              
==========================================
+ Hits        45937    45946       +9     
+ Misses      40993    40983      -10     
+ Partials       51       50       -1     

Impacted Files	Coverage Δ
ghost/mailgun-client/lib/mailgun-client.js	75.23% <79.16%> (+1.18%)	:arrow_up:
ghost/api-framework/lib/http.js	64.56% <0.00%> (ø)
ghost/admin/app/components/gh-image-uploader.js	94.66% <0.00%> (+0.66%)	:arrow_up:
ghost/admin/app/controllers/offer.js	42.32% <0.00%> (+0.82%)	:arrow_up:
ghost/admin/app/components/gh-file-uploader.js	91.72% <0.00%> (+2.25%)	:arrow_up:
ghost/admin/app/components/gh-site-iframe.js	50.00% <0.00%> (+4.00%)	:arrow_up:

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