[Buckinghamshire] Fix WFS lookups to use POST.

[dracos]

The server no longer accepts GET requests with a filter. [skip changelog] Fixes FD-4779

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.35%. Comparing base (acb92bb) to head (c2e6c00).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
perllib/FixMyStreet/Cobrand/Buckinghamshire.pm	66.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5293      +/-   ##
==========================================
- Coverage   82.35%   82.35%   -0.01%     
==========================================
  Files         413      413              
  Lines       32722    32719       -3     
  Branches     5241     5241              
==========================================
- Hits        26949    26945       -4     
- Misses       4224     4225       +1     
  Partials     1549     1549              

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