🧹 Update poetry and pyproject

[alexjrk]

Update Poetry to version 2.1.3, and update pyproject.toml to follow the new Python packaging standard: Writing pyproject.toml.
The specification we previously used is now deprecated.

Submitter checklist

• Recommended: Join WireMock Slack to get any help in #help-contributing or a project-specific channel like #wiremock-java
• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• The repository's code style is followed (see the contributing guide)
• Test coverage that demonstrates that the change works as expected
• For new features, there's necessary documentation in this pull request or in a subsequent PR to wiremock.org