[Generic] Which Progression does apply? Update Readme

Please clarify, which progression should be reflected in this repository.
Do we align with vanilla progression from back in the days or with Classic progression as Blizzard adjusts quite a few things compared to vanilla. Which one is chosen as primary in case of conflicts? This should be clear in the Readme file of this repository.

Trigger point is the revert https://github.com/azerothcore/mod-progression-system/pull/289 which now bases the progression on classic instead of vanilla retail.
Currently, this revert decision seems to be taken **not in a transparent way** as there is no secondary ticket or public discussion and it does conflict.
Given this change, a lot more modifications could be required. Most prominent modification would be disabling the random dungeon finder, to be in line with Classic progression instead of vanilla progression.

Read the repository README and the reverted pull request #289 first, then compare the vanilla and Classic progression references mentioned in the issue. Done means the maintainers have chosen and documented the primary progression and clarified how conflicts are handled; any related changes, such as dungeon finder behavior, should be identified for follow-up.

Written by the indexing model from the issue text.