Change `--font-weight-bold` to `--font-weight-semibold` and 600 value, introduce...

Change `--font-weight-bold` to `--font-weight-semibold` and 600 value, introduce new font weight variables (#24827)

There was some recent discussion about this in Discord `ui-design`
channel and the conclusion was that
https://github.com/go-gitea/gitea/issues/24305 should have fixed their
OS font installation to have semibold weights.

I have now tested this 601 weight on a Windows 10 machine on Firefox
myself, and I immediately noticed that bold was excessivly bold and
rendering as 700 because browsers are biased towards bolder fonts. So
revert this back to the previous value.