[Breaking change] Prevent disabling the spaces plugin

[legrego]

Change description

We would like to remove the ability to disable the Spaces plugin in 8.0 (xpack.spaces.enabled)

Which release will ship the breaking change?

8.0

Describe the change. How will it manifest to users?

Users who have explicitly disabled Spaces via xpack.spaces.enabled will no longer be able to do so.

How many users will be affected?

Telemetry indicates that this will impact a very small number of installations. Most administrators run with Spaces enabled, which is the default behavior.

What can users do to address the change manually?

N/A

How could we make migration easier with the Upgrade Assistant?

Indicate that they must enable the Spaces plugin in order to upgrade to 8.0

Are there any edge cases?

No.

Test Data

Testing with spaces disabled:

xpack.spaces.enabled: false

Cross links

#89584

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[alisonelizabeth]

Hi @legrego! I'm going to remove the Elasticsearch UI team label. Please consider using the deprecations service (#94845) provided by core to register this deprecation. All registered deprecations will be displayed in the Upgrade Assistant (to be implemented via #97159). Feel free to reach out to myself or the core team with any questions!