Use name: nightly in docs-versions.sh for HEAD (#9915)

* Use name: master in docs-versions.sh for HEAD

At the time when this list is generated (at release), `src/VERSION` is
typically equivalent to the most recent tagged version.
It should just be called master and not present the development version
(which may even change during its course).

* Use "nightly" as label

Co-authored-by: Jonne Haß <me@jhass.eu>

Co-authored-by: Jonne Haß <me@jhass.eu>

scripts/docs-versions.sh

@@ -8,8 +8,7 @@ awk '
   BEGIN {
     print "{"
     print "  \"versions\": ["
-    getline current_version < "src/VERSION"
-    printf "    {\"name\": \"" current_version "\", \"url\": \"/api/master/\", \"released\": false}"
+    printf "    {\"name\": \"nightly\", \"url\": \"/api/master/\", \"released\": false}"
   }
 
   {