Use CurrentReplicas as a minimum for fallback replicas

[rickbrouwer]

When you are running a large number of pods and something goes wrong, Keda uses a fallback (if configured). However, the difference between the fallback.replicas and the current number of replicas can be large.
"Something going wrong" can also be temporary, which means it can suddenly scale down considerably and then return to the correct number of replicas.
With this PR, I want to make sure that you can indicate that it should use the current number of replicas (useCurrentReplicasAsMinimum) as fallback. If the given fallback.replicas is higher than the current replicas, then the fallback.replicas will be used.

• A new parameter has been added to the fallback struct (useCurrentReplicasAsMinimum)
• An adjustment has been made in the fallback.go to retrieve the current number of replicas
• In the scale_scaledobjects.go there was code that determined what the current number of replicas was. This has been moved to the scale_resolvers.go
• Fallback modified to determine whether to fall back to the fallback.replicas or the current number of replicas

If for any reason this option is not desired, please let me know.

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6450
Docs: kedacore/keda-docs#1519
Relates to discussion: #6334