Search: Test deprecated field in mixed cluster
Update the test for the deprecated `_source_exclude` parameter to run
against 6.6.0+ now that we've backported the deprecation.
Relates to
"Deprecated _source_include and _source_exclude":
- skip:
- version: " - 6.99.99"
+ version: " - 6.5.99"
reason: _source_include and _source_exclude are deprecated from 6.6.0
features: "warnings"
