[IMP] simplify migration path construction with seq

Replace manual loop building version array with seq + readarray:

Before (4 lines):
  declare -a versions
  nb_migrations=$((FINAL_VERSION - ORIGIN_VERSION))
  for ((i = 0; i < nb_migrations; i++)); do
      versions[i]=$((ORIGIN_VERSION + 1 + i))
  done

After (1 line):
  readarray -t versions < <(seq $((ORIGIN_VERSION + 1)) "$FINAL_VERSION")

The seq command is purpose-built for generating number sequences,
making the intent clearer and the code more concise.

upgrade.sh

@@ -111,12 +111,7 @@ log_info "Original filestore copied."
 
 log_step "PATH OF MIGRATION"
 
-declare -a versions
+readarray -t versions < <(seq $((ORIGIN_VERSION + 1)) "$FINAL_VERSION")
-nb_migrations=$((FINAL_VERSION - ORIGIN_VERSION))
-
-for ((i = 0; i < nb_migrations; i++)); do
-    versions[i]=$((ORIGIN_VERSION + 1 + i))
-done
 log_info "Migration path is ${versions[*]}"