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.

web_src/js/components/DashboardRepoList.vue

@@ -74,7 +74,7 @@
             <a class="repo-list-link muted gt-df gt-ac gt-sb" :href="repo.link">
               <div class="item-name gt-df gt-ac gt-f1">
                 <svg-icon :name="repoIcon(repo)" :size="16" class-name="gt-mr-2"/>
-                <div class="text gt-bold truncate gt-ml-1">{{ repo.full_name }}</div>
+                <div class="text gt-font-semibold truncate gt-ml-1">{{ repo.full_name }}</div>
                 <span v-if="repo.archived">
                   <svg-icon name="octicon-archive" :size="16" class-name="gt-ml-2"/>
                 </span>

web_src/js/components/DiffFileList.vue

@@ -1,7 +1,7 @@
 <template>
   <ol class="diff-detail-box diff-stats gt-m-0" ref="root" v-if="fileListIsVisible">
     <li v-for="file in files" :key="file.NameHash">
-      <div class="gt-bold gt-df gt-ac pull-right">
+      <div class="gt-font-semibold gt-df gt-ac pull-right">
         <span v-if="file.IsBin" class="gt-ml-1 gt-mr-3">{{ binaryFileMessage }}</span>
         {{ file.IsBin ? '' : file.Addition + file.Deletion }}
         <span v-if="!file.IsBin" class="diff-stats-bar gt-mx-3" :data-tooltip-content="statisticsMessage.replace('%d', (file.Addition + file.Deletion)).replace('%d', file.Addition).replace('%d', file.Deletion)">