replaces one of the Trending themes

[sarah11918]

This is not urgent! Can merge next week sometime.

Replaces astrolus with upstart as a trending theme.

Notes:

• did not, but I'm sure we could, delete the image for astrolus
• I think I created an image with the wrong dimensions? I instictively used the ones for showcase, but this seems less tall than the others. A new image can be captured from https://upstart-astro.netlify.app/

Browser Test Checklist

I have tested this PR on at least three of the following browsers:

• Chrome / Chromium
• Firefox
• Android Firefox
• Safari
• iOS Safari

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