gnu: qtsvg: Rename variable to qtsvg-5.

This is in preparation of the qtsvg update to version 6.

This change was automated via the following command:

  git grep -l '\bqtsvg\b' | xargs sed 's/qtsvg/qtsvg-5/g' -i

gnu/packages/web-browsers.scm

@@ -454,7 +454,7 @@ access.")
              openssl
              qtbase-5
              qtmultimedia
-             qtsvg))
+             qtsvg-5))
       (home-page "https://kristall.random-projects.net")
       (synopsis "Small-internet graphical client")
       (description "Graphical small-internet client with with many features