refactor(apigateway): remove POWERTOOLS_EVENT_HANDLER_DEBUG env var

[heitorlessa]

Issue number: #1459

Summary

Changes

Please provide a summary of what's being changed

Remove warning and deprecate POWERTOOLS_EVENT_HANDLER_DEBUG in favour of POWERTOOLS_DEV before launching V2.

User experience

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

Replaced older env var with POWERTOOLS_DEV in docs too, and note about future deprecation.

Checklist

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• 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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[codecov-commenter]

Codecov Report

Base: 99.40% // Head: 99.36% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (a8fe423) compared to base (2864b46).
Patch coverage: 95.55% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #1620      +/-   ##
==========================================
- Coverage   99.40%   99.36%   -0.04%     
==========================================
  Files         127      127              
  Lines        5842     5808      -34     
  Branches      367      369       +2     
==========================================
- Hits         5807     5771      -36     
- Misses         17       18       +1     
- Partials       18       19       +1     

Impacted Files	Coverage Δ
aws_lambda_powertools/shared/constants.py	100.00% <ø> (ø)
aws_lambda_powertools/utilities/batch/base.py	97.79% <ø> (ø)
...s/utilities/data_classes/dynamo_db_stream_event.py	98.13% <95.45%> (-1.87%)	⬇️
aws_lambda_powertools/event_handler/api_gateway.py	100.00% <100.00%> (ø)

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[leandrodamascena]

The fastest PR on all github 🚀