Still do validation even when migrating to less than the current engine version

[rowanseymour]

Just realised we can still call ReadFlow so that we get structural validation.

1. Do the JSON to JSON migration (e.g. 11.9 to 13.0.0)
2. Call ReadFlow on the migrated JSON which internally will migrate to 13.1.0 but that we don't return the result of that so it doesn't matter

[codecov]

Codecov Report

Merging #209 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   47.24%   47.25%   +0.01%     
==========================================
  Files          81       81              
  Lines        7023     7021       -2     
==========================================
  Hits         3318     3318              
+ Misses       3182     3181       -1     
+ Partials      523      522       -1

Impacted Files	Coverage Δ
web/flow/flow.go	76.92% <100%> (+2.84%)	⬆️

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