info.xml: set compatibility back to 5.51

I have no reason to think it's not good even farther back.  Alice
tested compatibility at 5.53, but because forward compatibility is
implied, I wanted to make sure it included a few recent versions,
too.

info.xml

@@ -8,15 +8,15 @@
     <author>Andie Hunt</author>
     <email>[email protected]</email>
   </maintainer>
-  <releaseDate>2019-07-03</releaseDate>
-  <version>2.0</version>
+  <releaseDate>2022-08-11</releaseDate>
+  <version>2.1</version>
   <develStage>stable</develStage>
   <urls>
 	  <url desc="documentation">http://aghstrategies.com/counties</url>
     <url desc="Licensing">http://www.gnu.org/licenses/agpl-3.0.html</url>
   </urls>
   <compatibility>
-    <ver>5.53</ver>
+    <ver>5.51</ver>
   </compatibility>
   <comments>Use this extension to load counties for the United States.  After installation, this extension does nothing (and can even be disabled).</comments>
 </extension>