Switch to Python 3.15 by default in CI and Dockerfile

[rolandwalker]

Description

Switch to Python 3.15 by default in CI and Dockerfile, cleaning up a few other minor references such as test strings.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the mycli/AUTHORS.txt file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .