[skip-ci] Update UPGRADING file with new IMAP namespace\

Ayesh · Ayesh · commit 01934224bce3 · 2021-05-06T23:34:17.000+05:30
Followed by the bundled extension namespace RFC, php#6925 updated the `IMAPConnection` class to `IMAP\Connection`. This updates the UPGRADING file to reflect that change.
diff --git a/UPGRADING b/UPGRADING
@@ -63,7 +63,7 @@ PHP 8.1 UPGRADE NOTES
     instead of resources.
 
 - IMAP:
-  . The IMAP functions now accept and return, respectively, IMAPConnection objects
+  . The IMAP functions now accept and return, respectively, IMAP\Connection objects
     instead of resources.
 
 - LDAP:
