[Proposal] Remove timeline application plugin

[ananzh]

Background

The timeline application (not timeline visualization) was deprecated in ODFE 7.0 and OSD 1.0. Since then it is only achievable if add a config in opensearch_dashboards.yml to enable it. Currently, timeline application is still using angular framework, which has severe security concerns. Therefore, based on the research, we propose to remove timeline application.

Proposal

Below are the sub tasks.

Functional tests

• Understand timeline application functional tests. [Timeline] Document all the functional tests related to timeline application #3584
• Re-write/modify the tests to allow the functional checks to work with timeline visualization. [Timeline][Functional tests] Remove the functional tests only related to timeline application #3593

Remove timeline application.

Clean out

• clean legacy pipelines
• remove any advanced settings that are related to timeline application
• clean timeline sheets saved object related contents
• clean docs

Add document for timeline visualization

[kavilla]

I believe there are still community members using it. Do we have a replacement for them or are we deprecating it as a whole? If so I'd recommend making this a 3.0 thing and really letting users know that Timeline deprecation is taking place since it's been deprecated for so long and give them a migration path.

[joshuarrrr]

The migration path already existed from timeline to vis_type_timeline at the time of the fork, and timeline is hidden from the navigation from default. What will be hard deprecated is the idea of timeline "sheets" because dashboards were chosen as a more cohesive experience (and I agree with that choice).