Stop using `ryanluker.vscode-coverage-gutters` vscode plugin

As coverage is now builtin:
https://code.visualstudio.com/docs/python/testing#_run-tests-with-coverage

We don't need the `ryanluker.vscode-coverage-gutters` vscode plugin for this anymore

This ticket is to remove that, then check whether we still need to override the `cov.xml` output location anymore. If we don't then we can remove all the coverage output and all mention of coverage in `pyproject.toml` and use the recommended invocation for codecov:
https://github.com/codecov/example-python/blob/fe5346d9068be9f9e51eb2de73ccc375dbae6c4e/.github/workflows/ci.yml#L17-L24

This needs testing on a different project first before putting into copier

Start by locating the VS Code extension configuration and coverage-related entries in pyproject.toml, then compare the CI invocation with the linked Codecov Python workflow example. Test the proposed configuration in a different project before changing the copier template; done means the plugin and unnecessary coverage overrides or mentions are removed without breaking coverage reporting.

索引モデルが issue の本文から書いたものです。