CMS6: Flushable classes aren't flushed when flushing from sake

[GuySartorelli]

The Flushable interface implementors have flush() called on them when the system is flushed. This is currently handled in a middleware - but middleware isn't run during sake execution. This means in CMS 6 those classes do not get flushed when flushing from sake.

Acceptance Criteria

• sake flush and sake [command] --flush results in the flush() method being called on all Flushable implementors.

PRs

CMS 5 PRs

• API Deprecate FlushMiddelware #11444
• DOC Document deprecation developer-docs#608

CMS 6 PRs

kitchen sink CI run since this is the sort of change that could potentially bork tests

• FIX Ensure everything gets flushed when flushing from sake #11436
• DOC Document changes to when Flushable gets flushed developer-docs#607

[emteknetnz]

Merged CMS 5 PRs - you'll need to merge up