Merge branch 'eyraud/doc_ckpt_incompatibility' into 'master'

Reject older checkpoint versions

Closes #119

See merge request eng/cov/gnatcoverage!272

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

Closes eng/cov/gnatcoverage#119

Author: Jugst3r

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);