[backend] Invalid TAXII header when collection is exposed to public

[helene-nguyen]

Proposed changes

• Set header with mediat type application/taxii+json to be aligned with STIX 2.1 standard

Related issues

• [backend] Invalid TAXII header when collection is exposed to public #5621

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

Expected output

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (3153001) 64.72% compared to head (04dc9a1) 64.71%.
Report is 1 commits behind head on master.

❗ Current head 04dc9a1 differs from pull request most recent head f1d0753. Consider uploading reports for the commit f1d0753 to get more accurate results

Files	Patch %	Lines
...cti-platform/opencti-graphql/src/http/httpTaxii.js	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5755      +/-   ##
==========================================
- Coverage   64.72%   64.71%   -0.01%     
==========================================
  Files         516      516              
  Lines       60890    60894       +4     
  Branches     4855     4856       +1     
==========================================
  Hits        39410    39410              
- Misses      21480    21484       +4     

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