[mattijn]

This PR bumps the minimum vegafusion requirement to 2.0.3 and revert the temporary workaround from #3857.

Version requirement:

• Minimum vegafusion version is now 2.0.3 (was 1.6.9)
• Vegafusion 2.x no longer depends on vegafusion-python-embed

Simplified vegafusion import (altair/utils/_importers.py):

• Removed legacy version checking for vegafusion < 2.0.0
• Removed all references to vegafusion-python-embed package

Reverted temporary workaround (Reverts #3857):

• Restored jsdelivr CDN URLs for vega-datasets (was temporarily using GitHub Raw)
• Re-enabled 3 tests that were skipped: gapminder_bubble_plot.py, co2_concentration.py, interval_selection.py
• Updated dataset metadata files with correct jsdelivr URLs
• Fixed doctest example URL in altair/datasets/_data.py