Site updated to GSoC 2025 state

Did the following updates:

• Moved all non-selected GSoC 2024 ideas to the old ideas section.
• Moved all selected GSoC 2024 ideas to the past projects section.
• Updated the copyright year to 2025 and updated all mentions of GSoC 2024 to GSoC 2025.
• Created a page for old proposals, categorizing them by year with separate sections for accepted and rejected proposals.

You can view the updated version here