Update gdb/NEWS after GDB 12 branch creation.

This commit a new section for the next release branch, and renames
the section of the current branch, now that it has been cut.
diff --git a/gdb/NEWS b/gdb/NEWS
index c609da7..4308e17 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -1,7 +1,9 @@
 		What has changed in GDB?
 	     (Organized release by release)
 
-*** Changes since GDB 11
+*** Changes since GDB 12
+
+*** Changes in GDB 12
 
 * DBX mode is deprecated, and will be removed in GDB 13