chore(ci): improve feature flags UA

[leandrodamascena]

Issue number: #6334

Summary

Changes

Fixes the user agent header for feature flags to use feature_flags instead of the generic parameters label when creating boto3 clients.

User experience

Please share what the user experience looks like before and after this change

Checklist

If your change doesn't seem to apply, please leave them unchecked.

• Meet tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

Acknowledgment

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.26%. Comparing base (d017744) to head (0904ff3).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...da_powertools/utilities/feature_flags/appconfig.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7051      +/-   ##
===========================================
- Coverage    96.27%   96.26%   -0.01%     
===========================================
  Files          275      275              
  Lines        12912    12917       +5     
  Branches       954      955       +1     
===========================================
+ Hits         12431    12435       +4     
  Misses         376      376              
- Partials       105      106       +1     

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