Replace unstyled meter with progress (#19968)

Replace the only `<meter>` element in use with a `<progress>` which is
styled properly. Also slightly adjust colors on it for better contrast.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/_review.less

@@ -280,3 +280,8 @@ a.blob-excerpt:hover {
 .viewed-file-checked-form {
   background-color: var(--color-primary-light-4);
 }
+
+#viewed-files-summary {
+  width: 72px;
+  height: 10px;
+}