Fix deprecated calls to curl (move to curl MIME API)

[melroy89]

Migrating away from the deprecated curl_formadd to the new libcurl MIME API.

@vasild Could you please review this PR? It's a important one (I kept the old code for your reference, I will remove this before merge this PR)/

See "Http posting" heading: https://curl.se/libcurl/c/libcurl-tutorial.html#HTTP

Resolves: https://gitlab.melroy.org/libreweb/libreweb-browser/-/jobs/15893#L130

[codecov-commenter]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (535a1e3) 57.49% compared to head (7ce8722) 55.16%.

Files	Patch %	Lines
src/http/transport-curl.cc	32.14%	0 Missing and 19 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   57.49%   55.16%   -2.33%     
==========================================
  Files           2        2              
  Lines         447      455       +8     
  Branches      206      219      +13     
==========================================
- Hits          257      251       -6     
  Misses         17       17              
- Partials      173      187      +14     

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[vasild]

LGTM, you can drop the old commented code and squash.