Fix interval_range right child non-zero offset

[mroeschke]

Description

xref #16507

Similar to what is done in IntervalIndex.from_breaks, interval_index generates the right edges by slicing a range of fencepost edges. However, we don't want to maintain the new offset (1) on the right edge after slicing as it adversely impacts subsequent indexing operations.

Additionally, I noticed that Index(struct_data) would automatically convert it to an IntervalIndex, but IntervalIndex has a strict requirement on the data have left/right keys, so making this raise a NotImplementedError instead
^ Will tackle this in a follow up, looks like there are cases where this is valid

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mroeschke]

/merge