[RAC] [Observability] Enable the observability alerts, cases and ruleRegistry feature flags by default

[weltenwort]

📝 Summary

Currently the observability alerts and cases pages are hidden by default. Similarly, the alert index templates and indices are not created or updated and no alert documents are written on alert execution. The feature flags that control this should default to being enabled if we want this to be available to all users.

✔️ Acceptance criteria

• The feature flag xpack.ruleRegistry.write is set to true by default.
• The feature flags for the observability alerting and cases pages are set to true by default or removed.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[weltenwort]

@cyrille-leclerc Can you help us understand what the criteria for moving out of experimental status should be?

[jasonrhodes]

I'm also curious if we have settled on whether this will now be a BETA feature or just go right to GA from Experimental.

[weltenwort]

related APM documentation changes: #100039

[weltenwort]

@cyrille-leclerc Do we need any changes to the "experimental" callout when we change the feature flags to be enabled by default?

[cyrille-leclerc]

@vinaychandrasekhar @hbharding what do you think about this? Do we keep the experimental banner as it is today?

[vinaychandrasekhar]

@cyrille-leclerc @weltenwort Yes I would like to review and suggest a couple of changes. Will discuss with @hbharding and will circle back here. Thanks