Add Minnesota state flag

Added the SVG file for the Minnesota (us-mn) state flag and updated gallery.md to include Minnesota in the flag gallery.

Author: HatScripts

flags/us-mn.svg

@@ -381,6 +381,7 @@ https://hatscripts.github.io/circle-flags/flags/xx.svg
   <div><img src="flags/us-hi.svg" width="96"/><p>Hawaii (<code>us-hi</code>)</p></div>
   <div><img src="flags/us-in.svg" width="96"/><p>Indiana (<code>us-in</code>)</p></div>
   <div><img src="flags/us-md.svg" width="96"/><p>Maryland (<code>us-md</code>)</p></div>
+  <div><img src="flags/us-mn.svg" width="96"/><p>Minnesota (<code>us-mn</code>)</p></div>
   <div><img src="flags/us-mo.svg" width="96"/><p>Missouri (<code>us-mo</code>)</p></div>
   <div><img src="flags/us-ms.svg" width="96"/><p>Mississippi (<code>us-ms</code>)</p></div>
   <div><img src="flags/us-nc.svg" width="96"/><p>North Carolina (<code>us-nc</code>)</p></div>