Use azure disk caching defaults

[hebelsan]

What would you like to be added:
The documentation of the caching field is the following:

Specifies the caching requirements. Possible values are: None, ReadOnly, ReadWrite.
The defaulting behavior is: None for Standard storage. ReadOnly for Premium storage.

However we currently always set the caching option to NONE for both dataDisks and osDisks and therefore don't make use of these defaults:

machine-controller-manager-provider-azure/pkg/azure/provider/helpers/driver.go

Lines 771 to 774 in e914f57

	caching := armcompute.CachingTypesNone
	if !utils.IsEmptyString(specDataDisk.Caching) {
	caching = armcompute.CachingTypes(specDataDisk.Caching)
	}

If we change the behaviour in the MCM we can afterwards change the machineclass.yaml template in the provider-extension.
https://github.com/gardener/gardener-extension-provider-azure/blob/6cb84f21b9191f728319e82f66a7804b5818d871/charts/internal/machineclass/templates/machineclass.yaml#L101

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