Promote claim server-side apply to beta

[negz]

What problem are you facing?

In Crossplane v1.15 we updated the claim reconciler to use server-side apply to create and update XRs. This is currently off by default, behind an alpha feature flag - #5313.

How could Crossplane help solve your problem?

I'm not aware of any issues with the implementation - we should flip it to beta.

[negz]

Anecdotally, I'm seeing the E2E tests flake a lot for this one though. Something to address before beta:

TestPropagateFieldsRemovalToXRAfterUpgrade/TestPropagateFieldsRemovalToXRAfterUpgrade/FieldsRemovalPropagatedToXR

[negz]

Something to be aware of: crossplane-contrib/provider-kubernetes#246 (comment)

It seems like in some cases SSA will bump the resource version of the applied object even if nothing changed. In theory this could cause a tight loop of reconciles:

1. Claim controller uses SSA to apply XR
2. API server bumps XR resource version even though nothing changed
3. Claim controller notices updated XR, triggering a reconcile
4. Back to step 1