API: Change `assert_index_equal(exact=)` default from `"equiv"` to `True`

[mroeschke]

Recently, #57429 slipped through the cracks because Index[int64] is considered equal to RangeIndex. While convenient, this allows code changes to potentially introduce a change that leads to increase in memory usage (or decrease in the hopeful case). I don't think this lack of clarity is worth the convenience of making Index[int64] equivalent to RangeIndex by default and therefore exact should default to True

[rhshadrach]

+1