Remove the obsolete Azure Key Vault secret store extensions

[stijnmoreels]

Is your feature request related to a problem? Please describe.
When designing the Azure Key Vault secret provider for the secret store, there were a lot of changes and migrations: adding options, different authentication mechanisms, new Azure SDK... The older secret store extensions are still in the source.

Describe the solution you'd like
Remove the older obsolete Azure Key Vault secret store extensions.

[fgheysels]

I think we should do this, and not wait any longer with it.
All those obsolete methods (some of them are already obsolete for quite some time - for instance the ones that have the allowCaching parameter are really polluting the intellisense in Visual Studio. It will make things cleaner and easier if they're gone.

[stijnmoreels]

Yes, was one of the suggestions on observability too, but we didn't go through with it bc that was the only reason why we would have a breaking change.
In this case, we could also include removing the deprecated Azure Key Vault package, though.