Removed intermediate python versions in CI matrix

[perseoGI]

Changelog: omit
Docs: omit

Remove intermediate python versions in CI to save resources.
The minimum python version still supported by conan is 3.6.

Changes across python versions (simplified):

• Python 3.8 introduced := (walrus operator).
• Python 3.9+ deprecated collections.OrderedDict instantiation without from collections.
• Python 3.13 removes some legacy modules (e.g., cgi and optparse).

We will only test the oldest supported version (3.6) and the newest Python version (3.13).
This pipeline will ensure complete compatibility across all intervals; if a breaking change is introduced, one of the jobs will fail.

Close #17835

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.