Add --version flag to create_venvs.py to filter specific Airflow versions

[budaesandrei]

Issue #, if available: N/A

Description of changes:

Added --version flag to create_venvs.py to allow creating virtual environments for a specific Airflow version instead of processing all versions.

Motivation:
When working with a single Airflow version, the script would attempt to create venvs for all versions in images/airflow/, which is time-consuming and can fail if the system Python version is incompatible with older Airflow versions.

Changes:

• Added --version argument to filter which Airflow version to process
• Added validation to check if the specified version exists
• Displays available versions with helpful error message if invalid version is provided
• Updated README.md

Usage:

# Create venv for only 3.0.6
python3 create_venv.py --target development --version 3.0.6

# Create venvs for all versions (existing behavior)
python3 create_venv.py --target development

Testing:

• Tested with valid version (3.0.6)
• Tested with invalid version (displays error and lists available versions)
• Tested without version flag (processed all versions as before)

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