Resolve urllib3 conflict and update to v2.6.3

[lresende]

• Remove appengine-python-standard which is no longer needed with
  modern dependencies and restricted urllib3 to <2.
• Update urllib3 to >=2.5.0 in pyproject.toml to address CVEs.
• Update urllib3 to 2.6.3 in test requirements.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@a9ae5a4). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3374   +/-   ##
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  Coverage        ?   67.85%           
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  Files           ?       58           
  Lines           ?     3117           
  Branches        ?      499           
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  Hits            ?     2115           
  Misses          ?      810           
  Partials        ?      192           

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