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.

templates/repo/diff/box.tmpl

@@ -97,7 +97,7 @@
 											{{svg "octicon-chevron-down" 18}}
 										{{end}}
 									</button>
-									<div class="gt-bold gt-df gt-ac gt-mono">
+									<div class="gt-font-semibold gt-df gt-ac gt-mono">
 										{{if $file.IsBin}}
 											<span class="gt-ml-1 gt-mr-3">
 												{{$.locale.Tr "repo.diff.bin"}}

templates/repo/diff/comments.tmpl

@@ -11,7 +11,7 @@
 		<div class="ui top attached header comment-header gt-df gt-ac gt-sb">
 			<div class="comment-header-left gt-df gt-ac">
 				{{if .OriginalAuthor}}
-					<span class="text black gt-bold gt-mr-2">
+					<span class="text black gt-font-semibold gt-mr-2">
 						{{svg (MigrationIcon $.root.Repository.GetOriginalURLHostname)}}
 						{{.OriginalAuthor}}
 					</span>

templates/repo/issue/view_content.tmpl

@@ -23,7 +23,7 @@
 					<div class="ui top attached header comment-header gt-df gt-ac gt-sb" role="heading" aria-level="3">
 						<div class="comment-header-left gt-df gt-ac">
 							{{if .Issue.OriginalAuthor}}
-								<span class="text black gt-bold">
+								<span class="text black gt-font-semibold">
 									{{svg (MigrationIcon .Repository.GetOriginalURLHostname)}}
 									{{.Issue.OriginalAuthor}}
 								</span>

templates/repo/issue/view_content/comments.tmpl

@@ -27,7 +27,7 @@
 					<div class="ui top attached header comment-header gt-df gt-ac gt-sb" role="heading" aria-level="3">
 						<div class="comment-header-left gt-df gt-ac">
 							{{if .OriginalAuthor}}
-								<span class="text black gt-bold gt-mr-2">
+								<span class="text black gt-font-semibold gt-mr-2">
 									{{svg (MigrationIcon $.Repository.GetOriginalURLHostname)}}
 									{{.OriginalAuthor}}
 								</span>
@@ -420,7 +420,7 @@
 								{{end}}
 								<span class="text grey muted-links">
 									{{if .OriginalAuthor}}
-										<span class="text black gt-bold">
+										<span class="text black gt-font-semibold">
 											{{svg (MigrationIcon $.Repository.GetOriginalURLHostname)}}
 											{{.OriginalAuthor}}
 										</span>
@@ -548,7 +548,7 @@
 																{{end}}
 																<span class="text grey muted-links">
 																	{{if .OriginalAuthor}}
-																		<span class="text black gt-bold">
+																		<span class="text black gt-font-semibold">
 																			{{svg (MigrationIcon $.Repository.GetOriginalURLHostname)}}
 																			{{.OriginalAuthor}}
 																		</span>

templates/repo/sub_menu.tmpl

@@ -28,7 +28,7 @@
 			{{range .LanguageStats}}
 			<div class="item gt-df gt-ac gt-jc">
 				<i class="color-icon gt-mr-3" style="background-color: {{.Color}}"></i>
-				<span class="gt-bold gt-mr-3">
+				<span class="gt-font-semibold gt-mr-3">
 					{{if eq .Language "other"}}
 						{{$.locale.Tr "repo.language_other"}}
 					{{else}}