[Fleet] Kibana upgrade fails with saved object migration error

[juliaElastic]

Kibana version: 8.7.0 snapshot

Describe the bug: On upgrade (or making change to local kibana), kibana throws a fatal error coming from saved object migrations.

Expected behavior:
Kibana shouldn't crash.

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

 FATAL  Error: Unable to complete saved object migrations for the [.kibana] index. Please check the health of your Elasticsearch cluster and try again. Unexpected Elasticsearch ResponseError: statusCode: 400, method: PUT, url: /.kibana_8.7.0_001/_mapping?timeout=60s error: [illegal_argument_exception]: can't merge a nested mapping [epm-packages.installed_kibana] with a non-nested mapping,

Any additional context:
Likely caused by this change: #148141

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

As discussed on slack with Dmitrii, this only happens in local, and not going to impact real upgrades. As a workaround we can purge the contents of ES locally.