BWC failures due to system index access deprecation wranings on old cluster

[gwbrown]

Build scan: https://gradle-enterprise.elastic.co/s/4dinnv2nw6t6w

Related to #60945 - when that PR was backported to 7.x, it started causing deprecation warnings to be emitted during the "old cluster" phase of upgrade tests in master, which are not yet handled by those tests.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Core)