Set `--font-weight-bold` to 600 (#24840)

Backport of https://github.com/go-gitea/gitea/pull/24827 to 1.19, just
the font weight reduction.

web_src/css/base.css

@@ -3,10 +3,7 @@
   --fonts-proportional: -apple-system, "Segoe UI", system-ui, "Roboto", "Helvetica Neue", "Arial";
   --fonts-monospace: "SFMono-Regular", "Menlo", "Monaco", "Consolas", "Liberation Mono", "Courier New", monospace, var(--fonts-emoji);
   --fonts-emoji: "Apple Color Emoji", "Segoe UI Emoji", "Noto Color Emoji", "Twemoji Mozilla";
-  /* "font-weight: bold" starts from 700, some fonts do not provide "bolding" for weight 600. */
-  /* But some users consider "700" is too heavy, so use 601, which is when Segoe UI on Linux */
-  /* starts bolding. */
-  --font-weight-bold: 601;
+  --font-weight-bold: 600;
   /* backgrounds */
   --checkbox-mask-checked: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="-1 -1 18 18" width="16" height="16"><path fill-rule="evenodd" d="M13.78 4.22a.75.75 0 010 1.06l-7.25 7.25a.75.75 0 01-1.06 0L2.22 9.28a.75.75 0 011.06-1.06L6 10.94l6.72-6.72a.75.75 0 011.06 0z"></path></svg>');
   --checkbox-mask-indeterminate: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16"><path fill-rule="evenodd" d="M2 7.75A.75.75 0 012.75 7h10a.75.75 0 010 1.5h-10A.75.75 0 012 7.75z"></path></svg>');