Docs: Change `mjs` codeblocks to `js` (#79163)
Closes:
https://linear.app/vercel/issue/DOC-4450/missing-syntax-highlighting-on-mjs-codeblocks
It looks like shiki doesn't really support `mjs` code blocks. For now,
we keep the filename with `.mjs` extension, but change to `js` code
block type.
Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>
