sponsor: Add Flutter Enterprises as Silver sponsor

Flutter Enterprises is a sponsor on OpenCollective, the image and
alt attribute text was supplied by them via email.

_sponsors.html

@@ -122,6 +122,7 @@
 </a></div>
 <!-- Silver-level but opencollective doesn't show (?) -->
  <div class="silver"><p> <a class="x" href="https://www.monkeybreadsoftware.de/" rel="sponsored"><img src="pix/silver/monkeybreadsoftware-2.jpg" alt="monkeybreadsoftware"></a></div>
+ <div class="silver"><p> <a class="x" href="https://casino.xyz/" rel="sponsored"><img src="pix/silver/casinoxyz.png" alt="casino.xyz"></a></div>
 <!-- Silver for curl on github -->
  <div class="silver"><p> <a class="x" href="https://www.celebian.com/" rel="sponsored"><img src="pix/silver/celebian.png" alt="celebian"></a></div>
 <!---- Silver end ----->