#25306 REMOVE support to python3.10

Description

This PR removes Python 3.10 support from the Python generators, as it reached End of Life (EOL) on October 1, 2026.

Changes

Removed Python 3.10 from the supported Python versions.

Updated the minimum supported Python version to 3.11.

Updated the relevant generator configurations, documentation, and samples.

Related issue

Closes #25305

Additional context

Official Python version status: https://devguide.python.org/versions/

IMPORTANT:
This MR has part of code of this MR https://github.com/OpenAPITools/openapi-generator/pull/25304

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Removes Python 3.10 support from the Python client and FastAPI server generators because it reached end of life. Minimum required version becomes 3.11, and CI matrices now run 3.11 through 3.15.

Test adjustments

• Adapts test_http_signature.py samples to a pycryptodome API change: exports RSA keys with format='PEM' and pkcs=1, and matches either of two possible error messages.