Related to #39250 and #39236.

The purpose of the change here is to unify inconsistent behavior on the
merging.

For now, mergee side condition is replaced by merger side condition only
if both arel nodes are Equality or In clauses.

In other words, if mergee side condition is not Equality or In clauses
(e.g. `between`, `or`, `gt`, `lt`, etc), those conditions will be kept
even on the same column.

This behavior is harder to predict unless people are familiar with the
merging behavior.

Originally I suppose this behavior is just an implementation issue
rather than an intended one, since `unscope` and `rewhere`, which were
introduced later than `merge`, works more consistently.

Since most of the conditions are usually Equality and In clauses, I
don't suppose most people have encountered this merging issue, but I'd
like to deprecate the inconsistent behavior and will completely unify
that to improve a future UX.

```ruby
# Rails 6.1 (IN clause is replaced by merger side equality condition)
Author.where(id: [david.id, mary.id]).merge(Author.where(id: bob)) # => [bob]

# Rails 6.1 (both conflict conditions exists, deprecated)
Author.where(id: david.id..mary.id).merge(Author.where(id: bob)) # => []

# Rails 6.1 with rewhere to migrate to Rails 6.2's behavior
Author.where(id: david.id..mary.id).merge(Author.where(id: bob), rewhere: true) # => [bob]

# Rails 6.2 (same behavior with IN clause, mergee side condition is consistently replaced)
Author.where(id: [david.id, mary.id]).merge(Author.where(id: bob)) # => [bob]
Author.where(id: david.id..mary.id).merge(Author.where(id: bob)) # => [bob]
```