Consolidate older versions into 1.6 code path

[diofeher]

We should consolidate these two versions into the earliest OpenTofu version: 1.6.

There are multiple problems using 0.12 and 0.13:

• Semantically, there are a lot of code paths for handling special cases for these versions;
• When something is changed for the schemas being merged, we need to update tons of tests cases, e.g.: 53593ef