release-24.2: schemachanger: add PartitionName to IndexZoneConfig attr

[annrpom]

This patch adds a PartitionName attribute to the IndexZoneConfig
element to uniquely identify partitions from one another. Prior to this,
hitting a certain DROP path on a table with more than 1 partition on an
index would panic with an undropped backref error due to our inability
to distinguish these partitions from eachother.

Our code to drop these partition elements would always refer to the
top-most partition element in the descsCache's elementIndexMap
-- as the key to identify partition1 from partition2 was the same. So,
partition1 would be marked toAbsent "twice"; while partition2 got
ignored </3.

Another thing to note is that this bug is not just met for partitioned
tables of the nature mentioned above -- as the builderState's descriptor
cache would need to be unchanged between each partition's drop.

Epic: none
Fixes: #131862

Release note (bug fix): Addressed a bug with DROP CASCADE that would
occasionally panic with an undropped backref message on partitioned
tables.

---

Release justification: low-risk bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[fqazi]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rafiss)

[rafiss]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @annrpom)