Update pyproject.toml in line with Poetry 2.x

[anoadragon453]

Our pyproject.toml was generating a number of errors in our CI. This appears to be a follow-on from #18251. I noticed this when debugging an unrelated PR.

Click to see error log

Run echo "dir=$(python -m poetry env info -p || echo $(pwd)/.venv)" >> "$GITHUB_OUTPUT"
The "poetry.dev-dependencies" section is deprecated and will be removed in a future version. Use "poetry.group.dev.dependencies" instead.
Run actions/cache@v3
Cache not found for input keys: poetry-venv-Linux-3.13.3-75a3531907848fcb5135e329bb1b67981e783f587bd06c529d75259b473c2d49-
Run poetry check --lock
The "poetry.dev-dependencies" section is deprecated and will be removed in a future version. Use "poetry.group.dev.dependencies" instead.
Error: pyproject.toml changed significantly since poetry.lock was last generated. Run `poetry lock` to fix the lock file.
Warning: The "poetry.dev-dependencies" section is deprecated and will be removed in a future version. Use "poetry.group.dev.dependencies" instead.
Warning: [tool.poetry.name] is deprecated. Use [project.name] instead.
Warning: [tool.poetry.version] is set but 'version' is not in [project.dynamic]. If it is static use [project.version]. If it is dynamic, add 'version' to [project.dynamic].
If you want to set the version dynamically via `poetry build --local-version` or you are using a plugin, which sets the version dynamically, you should define the version in [tool.poetry] and add 'version' to [project.dynamic].
Warning: [tool.poetry.description] is deprecated. Use [project.description] instead.
Warning: [tool.poetry.readme] is set but 'readme' is not in [project.dynamic]. If it is static use [project.readme]. If it is dynamic, add 'readme' to [project.dynamic].
If you want to define multiple readmes, you should define them in [tool.poetry] and add 'readme' to [project.dynamic].
Warning: [tool.poetry.license] is deprecated. Use [project.license] instead.
Warning: [tool.poetry.authors] is deprecated. Use [project.authors] instead.
Warning: [tool.poetry.classifiers] is set but 'classifiers' is not in [project.dynamic]. If it is static use [project.classifiers]. If it is dynamic, add 'classifiers' to [project.dynamic].
ATTENTION: Per default Poetry determines classifiers for supported Python versions and license automatically. If you define classifiers in [project], you disable the automatic enrichment. In other words, you have to define all classifiers manually. If you want to use Poetry's automatic enrichment of classifiers, you should define them in [tool.poetry] and add 'classifiers' to [project.dynamic].
Warning: [tool.poetry.repository] is deprecated. Use [project.urls] instead.
Warning: [tool.poetry.extras] is deprecated. Use [project.optional-dependencies] instead.
Warning: Defining console scripts in [tool.poetry.scripts] is deprecated. Use [project.scripts] instead. ([tool.poetry.scripts] should only be used for scripts of type 'file').

Original GHA workflow run logs: https://github.com/element-hq/synapse/actions/runs/14931031372/job/41947086455?pr=18417#step:3:67

This PR fixes those issues by updating our pyproject.toml file to make use of the new keys defined by PEP 621.

Paired with matrix-org/sytest#1403 which updates poetry-core to 2.1.3 in Sytest.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)