Restructure scikit-build version metadata configuration

[steffengraber]

• Moved tool.scikit-build.metadata.version settings to inline tool.scikit-build section
• Updated version regex pattern
• Version will still be read from VERSION file
• No functional changes to version detection logic

[terhorstd]

Thanks for taking care of this section.

• Could you provide a reason why this change is necessary?
• If the section is split off, please move it under the [tool.scikit-build] section, not above.