internal/proxy: Compute stdlib latest version

The latest Go toolchain version does not have to match the latest Go
stdlib version. Manually compute the latest version instead.

Update dependencies

Update go.mod to Go 1.21

internal/server: Reorder platforms

internal/stdlib: Remove testdata

Update dependencies

go mod tidy

internal/stdlib: Remove unused code

internal/proxy: Use Go toolchains to fetch stdlib

internal/server: Log platform that was fetched

internal/server: Reduce number of platforms

Update about templates

internal/server: Fix error status code

internal/server: Disallow fetching of large modules

Update templates

I had first noticed that the <input> for searching was just a regular
`type="text"` instead of perhaps the more appropriate `type="search"`,
and then figured I'd throw godocs.io into https://validator.w3.org/nu/
to see if there was anything to fix up while I was here. Noticed a few
things:

* Removed an unnecessary trailing slash in <link>
* Removed the unnecessary `role="main"` on <main>
* Change <td>s to use CSS for alignment[1]
* Properly close the footer tag

Think these are all pretty clear improvements, but feel free to cut as
you see fit.

[1]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/td#align

internal/server: Limit file size to 30MB

templates: Fix example groups

internal/server: Fix package example play URLs

site.css: Update fonts

This somewhat mimics the behavior of Bootstrap 3.

site.css: Minor tweaks