fix: Avoid use of TLS < v1.2

[cpswan]

Fixes https://github.com/atsign-foundation/at_python/security/code-scanning/13

- What I did

Enforce minimum version of TLS 1.2

- How I did it

agy prompt:

codeql has identified an issue with at_client/connections/atconnection.py:88
where insecure SSL/TLS protocol is allowed. Make a recommendation for fixing
Don't execute any git commands. I've already created a feature branch and will
take care of a pull request once satisfied with the proposed change

- How to verify it

CI tests

- Description for the changelog

fix: Avoid use of TLS < v1.2