improve fallback mechanism for source version matching

[PeterMocary]

This is the improvement change originating from PR#1443, @pirat89 wanted me to create this PR so we could discuss it again.

I did not try to run it after the big rebase yet.

When searching in the upgrade paths map by the system version, we had implemented a fallback that allowed the upgrade process to find a target version even if the minor version of the system did not match exactly with the ones defined in the map.

The fallback mechanism was implemented using additional definitions in the map which did not have a minor version explicitly defined and thus would be only used as a fallback when the system version did not match any minor version.

This patch changes this fallback mechanism to instead search in the map by the latest minor version of the system's major version. The additional records are removed from the map as well.

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[PeterMocary]

It will need some changes, due to the big changes that happened recently.