Cross Cluster Search from 7.x to 6.8 can't create index pattern

[LeeDr]

Kibana version: 7.15.0 (7.x)

Elasticsearch version: 7.15.0 (7.x)

Server OS version: Ubuntu 16.04

Browser version: Chrome

Browser OS version: Ubuntu 21.04

Original install method (e.g. download page, yum, from source, etc.): tar.gz archives

Describe the bug: We had partial support for this range of mixed version cross cluster search up until at least 2021-07-28 but that level of support stopped working by 2021-08-10 (I haven't been able to narrow it down any closer yet)

Steps to reproduce:

1. Install Elasticsearch and Kibana from current 7.x branch
2. Install an Elasticsearch node of 6.18.x (I used 6.8.15, but the latest is 6.8.18)
3. configure the 6.8 node to be a remote cluster
4. load data in the 6.8 cluster
5. try to create an index pattern in 7.x for data in the 6.8 cluster

Expected behavior: There's ongoing discussion about what range of versions we can support with CCS.

Screenshots (if relevant):
On 7.x snapshot build from July 28th 7.15.0-2cefb6a4 I can create an index pattern using Cross Cluster Search to a remote "data" cluster running 6.8.15. And Discover shows data (although with known issues).

But on the 7.x snapshot build from August 10th 7.15.0-83f2fe8a, only the local remote cluster index pattern works. The remote "data" cluster on 6.8.15 doesn't.

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-stack-management (Team:Stack Management)

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[cjcenizal]

As this pertains to Index Patterns I'm transferring this from Stack Monitoring to App Services.

[LeeDr]

@mattkime zipped har file

localhost.har.zip