Set --gardenlet-uses-gardener-node-agent=true by default

[rfranzke]

How to categorize this PR?

/area os
/kind enhancement
/os ubuntu

What this PR does / why we need it:
Set --gardenlet-uses-gardener-node-agent=true by default (even when UseGardenerNodeAgent is not provided via .Values.gardener.gardenlet.featureGates in the Helm chart values).

Which issue(s) this PR fixes:
Part of gardener/gardener#8023
Follow-up of #99

Special notes for your reviewer:
This is a preparation step needed for removing the UseGardenerNodeAgent feature gate from gardener/gardener. Without this PR, when the feature gate is removed, it's no longer available via the .Values.gardener.gardenlet.featureGates in the Helm chart. The consequence would be that --gardenlet-uses-gardener-node-agent gets set to false. This obviously does not work with versions of gardener/gardener which no longer have the UseGardenerNodeAgent feature gate.

Release note:

It is assumed that `gardenlet`'s `UseGardenerNodeAgent` is turned on by default if the feature gate is not explicitly set. Hence, make sure to use at least Gardener `v1.82` when using this extension version.

[MrBatschner]

/lgtm