Update Python version to latest stable (3.14.0)

[marcieltorres]

Upgrade the project from Python 3.11 to the latest stable Python version 3.14.0 to take advantage of performance improvements, new features, and security updates.

Tasks:

• Update pyproject.toml python dependency from ^3.11 to ^3.14
• Update Dockerfile base image from python:3.11-slim to python:3.14-slim
• Test compatibility with existing dependencies (slack-bolt, aiohttp, pytest, ruff)
• Update development environment setup instructions
• Update README documentation to reflect new Python version requirement
• Add new section in README explaining how to upgrade/downgrade Python version for local development
• Update any Python version references in documentation

Acceptance Criteria:

• All tests pass with Python 3.14
• Docker build succeeds with new Python version
• README includes Python version upgrade/downgrade instructions
• Documentation accurately reflects Python 3.14 requirement

This upgrade ensures the project stays current with the latest Python improvements and maintains long-term security and performance benefits.