feat(node): make the zone label configurable

[lukasmetzner]

Adds HCLOUD_INSTANCES_ZONE_LABEL_ENABLED (default true), which controls
whether topology.kubernetes.io/zone / failure-domain.beta.kubernetes.io/zone
are set on Cloud Server nodes during initialization.

The default stays true: topology labels are only applied at node
initialization, so disabling this on a running cluster leaves a mix of labeled
and unlabeled nodes. docs/guides/zone-label.md covers the migration and how
to check whether anything selects on the label. We can remove this feature and
the label in the next major version.

Robot servers are unaffected and keep their datacenter zone label.

Closes #1146

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.25%. Comparing base (926a08a) to head (59c0813).

Files with missing lines	Patch %	Lines
internal/config/config.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1311      +/-   ##
==========================================
- Coverage   71.85%   71.25%   -0.61%     
==========================================
  Files          27       27              
  Lines        2800     2804       +4     
==========================================
- Hits         2012     1998      -14     
- Misses        613      622       +9     
- Partials      175      184       +9     

Flag	Coverage Δ
e2e	44.73% <71.42%> (+<0.01%)	⬆️
unit	67.61% <71.42%> (-0.60%)	⬇️

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