[drake_poetry] Upgrade to Poetry 2.0 standards

[tyler-yankee]

The drake_poetry example was created right around the time that Poetry released their 2.0, with a set of new standards for how pyproject.toml files should be structured. Our current pyproject.toml file is fine, but the dependabot can't parse it to bump the supported version of Drake when a new release comes out. And besides, we should keep things up to date for Poetry/Drake end users.

• Update the pyproject.toml to be formatted according to the new standards (notably, should use the [project] table).
• Update the supported version of Drake to match the convention in drake_pip. This may require some research, but it seems that Poetry now supports ~=, so we don't have to use ^.
• Update dependabot.yml as necessary to bump pyproject.toml. We may also be able to use the versioning-strategy field.

[jwnimmer-tri]

(FYI I transferred this issue over from the Drake repo into this repo.)