feat: update python to 3.11 and add build-arg

[LucasRoesler]

Update the default python 3 version to 3.11. Additionally, add a build arg for PYTHON_VERSION and WATCHDOG_VERSION so make it easy for developers to pin the python version that they need for their project.

The docs are updated with instructions about pinning the python version.

Motivation and Context

• [ ] I have raised an issue to propose this change (required)
• [ ] My issue has received approval from the maintainers or lead with the design/approved label We were discussing this during a Community Call in the context of AWS lambda and their slow upgrade cycle for python. We realized we also needed to update our templates.

How Has This Been Tested?

This was manually tested and working with the default "echo" function for each template.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[alexellis]

Looks really good, could we drop WATCHDOG_VERSION for this version please?

[LucasRoesler]

done