No longer require pyproject extra of scikit-build-core

Since `scikit-build-core` 0.11, the `pyproject` extra has no effect and
is provided only for backwards-compatibility. Update to
`scikit-build-core>=0.11` without the `pyproject` extra, but do so
conditionally for now, since we still advertise support for the
end-of-life Python versions 3.7 and 3.8, and `scikit-build-core` 0.11
requires Python 3.9.

This corresponds to the combination of
rordenlab/dcm2niix@601568f
and
rordenlab/dcm2niix@c26a6aa.

Author: musicinmybrain

pyproject.toml

@@ -1,5 +1,9 @@
 [build-system]
-requires = ["setuptools_scm>=7", "scikit-build-core[pyproject]>=0.5"]
+requires = [
+  "setuptools_scm>=7",
+  "scikit-build-core>=0.11; python_version >= '3.9'",
+  "scikit-build-core[pyproject]>=0.5; python_version < '3.9'",
+]
 build-backend = "scikit_build_core.build"
 
 [tool.scikit-build]