Use SPDX license tags in pyproject.toml

[bdice]

We want to use SPDX license identifiers in pyproject.toml files, following the Python Packaging Docs on the license field.

Today we define our licenses in a way that was meant to pass wheeltamer, an internal check for wheels which is no longer used.

Therefore we should replace this:

[project]
license = { text = "Apache 2.0" }

with this (note the dash, which makes this a valid SPDX identifier):

[project]
license = "Apache-2.0"

Specifically, this change conforms to: https://peps.python.org/pep-0639/#deprecate-license-key-table-subkeys