footer: tweak boldness of headings a bit

In commit d8b10296 ("footer: use h6 instead of strong"), I opted for a bit more semantical HTML, but it seems that dropping the strong-tag here had an unexpected side-effect; the boldness might have just been a bit less on my system, but it seems on other systems it's a lot less bold. In fact, not bold at all.

It turns out, this depends on the fonts installed on the system, and Bootstrap uses a default font-weight of 500 for headings. This looks fine most places, but in the footer here, we really want the headings to stand out from the rest.

So let's add back some boldness. By doing this in the CSS instead, we get to keep the semanticness of the HTML without having to sacrifice the look. The value of 600 was picked by a bit of trial-and-error; both systems I have where this doesn't render well as-is is fixed at 550 and above, but let's bump it a bit more to be sure.