Reject older checkpoint versions

We actually document that we reject older checkpoint versions but this
was not reflected in the code. Fix this.

Author: Jugst3r

Diff for: tools/gnatcov/checkpoints.adb

@@ -573,7 +573,7 @@ package body Checkpoints is
          if CP_Header.Magic /= Checkpoint_Magic then
             Fatal_Error ("invalid checkpoint file " & Filename);
 
-         elsif CP_Header.Version not in Checkpoint_Version then
+         elsif CP_Header.Version /= Checkpoint_Version'Last then
             Fatal_Error
               ("invalid checkpoint version" & CP_Header.Version'Img);