Fix no edge functions deployed to netlify

[andremralves]

Changes

fixes #6506

• This commit changes the generated edge function file from entry.mjs
  back to entry.js
• Apparently Netlify doesn't identify edge functions when the file extension is .mjs
• The issue started in version 2.1.3 when the file extension was changed.
• The extension was changed because of this issue Vercel Serverless - astro build fails with "Cannot use import statement outside a module" #6299 that was affecting Vercel and maybe could affect Netlify.
• But I tested version 2.1.2 of @astrojs/netlify using type: "commonjs" inside package.json and found no errors.

Testing

I just changed one test that was expecting .mjs.

Docs

n/a

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[bluwy]

Thanks for catching this! (And sorry for causing the regression)