gsoc: Add remaining milestone dates to GSoC 2026 Key Dates section

[alikhere]

This PR updates the GSoC 2026 → Key Dates section by adding missing milestones that are part of the official GSoC timeline.

The following milestones have been added:

• Community Bonding Period
• Coding Begins
• Midterm Evaluations
• Coding Ends
• Final Evaluations

These updates ensure the Kubeflow GSoC 2026 page is aligned with the official GSoC timeline and provides complete information to prospective contributors.

See the official gsoc timeline here: https://developers.google.com/open-source/gsoc/timeline

Before

After

Closes #4320

[google-oss-prow]

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[google-oss-prow]

Hi @alikhere. Thanks for your PR.

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[Arhell]

/ok-to-test