[7.x] Add deprecation message for xpack.spaces.enabled

[watson]

Fixes part of #82467

This PR adds a deprecation message if xpack.spaces.enabled is set to false in kibana.yml.

We could have expanded the scope and also shown the deprecation message if xpack.spaces.enabled was set to true, but to lessen the burden of upgrading to 8.0, we have decided that just checking for false is enough.

Here's how it will look in the Upgrade Assistant UI:

Release note

Deprecates disabling the Spaces plugin.

[kibanamachine]

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cc @watson

[legrego]

We added this deprecation back in the 7.11 release (#83984). Is that not working the way we thought it would?

[watson]

Ah you're right. I just realise that I've been working in the wrong file 🤦 - It should obvious go in the spaces plugin and not the security plugin. However, I can see that the deprecation warning added for spaces back then needs to be updated to align with our new guidelines. I'll close this PR and create a new one that changes that.

[legrego]

@watson thanks. For consistency, we may want to target your new PR against master, since this deprecation message lives there as well. Either that, or we open a different PR to remove the message from master altogether (whichever y'all think is best)

[watson]

Here's the new PR: #112242

I'll make a separate PR to remove the deprecation message in master.